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#4117154 - 08/04/21 03:43 AM P71 Rear Defrost
STR Offline
Stranger

Registered: 07/09/21
Posts: 6
Loc: TX
Been trying to figure out my rear defrost. I found out after I ripped off the C pillar trim that the driver's side has been hit and repaired. Badly. The bodywork looks perfect on the outside, but the window frame itself is a mess (it looks like they hooked onto the C pillar to pull the body straight, and the trunk floor has ripples on that side.. nothing on Carfax of course).

Does power come in to the defrost grid on the passenger or driver's side on a 2007? The wire coming off of the driver's side (black or grey with yellow stripe) is cut a few inches into the plastic panel. Throwing a meter on the passenger side terminal and using the door latch (on the pillar) as a ground, with the defrost turned on, gives 0 volts, so if power comes in from that tab, there's no power. I've already checked fuses and swapped relays around with known good ones; the light in the switch does come on, and I hear a click somewhere in the dash when I try to turn it on.

The accident damage appears to be to the LR door and C pillar; reaching into the quarter panel from the trunk, everything feels smooth. Whoever did the body work (aside from paint) did a great job at hiding everything. :/

I did look several pages back, for what it's worth. I'll probably see if my library has a Mitchell book for the car soon; this is the only real issue I still have with it. The AC causes the back window to frost up on really humid days (look I'm a bit hot blooded okay, don't judge me for running it on max AC when it's only 90 outside!), it'd be nice to have the defrost working.


The pic makes it look like there may be a fusible link in the cut wire, but it's not doing much good if it's cut.

e: radio reception, with both the factory AM/FM radio, and my aftermarket Pioneer head unit, are good, so the antenna portion is working to some degree.


Edited by STR (08/04/21 03:50 AM)

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#4117172 - 08/04/21 10:59 AM Re: P71 Rear Defrost [Re: STR]
RF_Overlord Offline

Poobah

Registered: 12/14/02
Posts: 7676
Loc: MA
Originally Posted By STR
Does power come in to the defrost grid on the passenger or driver's side on a 2007?
Both. You need to place your meter leads on both the left and right tabs. My 2003 factory wiring manual doesn't specify which tab is which polarity.

If there's no voltage, then you need to check the power in to the antenna/defroster module which is behind the passenger side C pillar or, depending on year, may be on the package shelf between the speakers.
_________________________
'03 Mercury Marauder (The Blackbird)
Trilogy Motorsports supercharged #61
Too many mods to list / Driveway Queen

'04 Grand Marquis LS Limited Edition (Spruce Green)
Daily Driver (Thanks, Adam!)

'00 Grand Marquis GS (Silver)
Rotted frame and broken transmission crossmember
Being resurrected...slowly.

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#4117192 - 08/04/21 03:15 PM Re: P71 Rear Defrost [Re: STR]
Davidzq Offline
Climber

Registered: 01/19/15
Posts: 567
Loc: Seattle, WA
Power is applied to the passenger side tab.

The driver side is ground, and the wire should be screwed into the C pillar where the patchwork is. But you may ground it by any means you prefer.
_________________________
David Hensley
Member of Northwest Panthers Car Club

2008 P71, Norsea Blue SAP
-Message Center cluster retrofit
-Towncar Signature L heated power seats
-Long overhead console
-Compustar remote start/security

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#4117196 - 08/04/21 03:40 PM Re: P71 Rear Defrost [Re: STR]
RF_Overlord Offline

Poobah

Registered: 12/14/02
Posts: 7676
Loc: MA
Thanks for the clarification, David. bow
_________________________
'03 Mercury Marauder (The Blackbird)
Trilogy Motorsports supercharged #61
Too many mods to list / Driveway Queen

'04 Grand Marquis LS Limited Edition (Spruce Green)
Daily Driver (Thanks, Adam!)

'00 Grand Marquis GS (Silver)
Rotted frame and broken transmission crossmember
Being resurrected...slowly.

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#4117624 - 08/09/21 08:42 AM Re: P71 Rear Defrost [Re: RF_Overlord]
STR Offline
Stranger

Registered: 07/09/21
Posts: 6
Loc: TX
Thank you David! I'll find a suitable ground, then troubleshoot further. I'd already tried swapping relays and checking fuses before I found that.... mess.

The protective rubber cap was still on the connection on the glass, so nobody even bothered trying to hook it up after the window was replaced. I found that lead sticking out from behind the trim, tried hooking it up, then removed the trim and found that it was cut...

Originally Posted By RF_Overlord

If there's no voltage, then you need to check the power in to the antenna/defroster module which is behind the passenger side C pillar or, depending on year, may be on the package shelf between the speakers.


I haven't checked the antenna module yet, but is it safe to assume it's getting power if I have no issue with radio stations? Everything comes in clear, both with the factory radio and the aftermarket Pioneer I have in there (which does digital HD radio - every local station that broadcasts in digital comes in as digital, no flip-flopping back and forth between digital and analog). There's a connector going from the C pillar to what looks like the antenna, but I haven't removed the trim on the passenger side.

I was checking between the passenger side tab and a known good ground, with key on/engine off and the defroster switch turned on (light in it was lit up, anyway). There's also nothing between the speakers on the package shelf, so I'm going to assume it's behind the pillar cover on the 07.

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#4117642 - 08/09/21 09:43 AM Re: P71 Rear Defrost [Re: STR]
RF_Overlord Offline

Poobah

Registered: 12/14/02
Posts: 7676
Loc: MA
Originally Posted By STR
There's also nothing between the speakers on the package shelf, so I'm going to assume it's behind the pillar cover on the 07.
I don't remember the year the location switch was made, but I do believe you're correct.
_________________________
'03 Mercury Marauder (The Blackbird)
Trilogy Motorsports supercharged #61
Too many mods to list / Driveway Queen

'04 Grand Marquis LS Limited Edition (Spruce Green)
Daily Driver (Thanks, Adam!)

'00 Grand Marquis GS (Silver)
Rotted frame and broken transmission crossmember
Being resurrected...slowly.

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#4117672 - 08/09/21 03:08 PM Re: P71 Rear Defrost [Re: STR]
Tenevic_P71 Offline
Over the Hill

Registered: 08/27/15
Posts: 1102
Loc: US
Antenna module is behind passenger C pillar cover on 06+. Or perhaps they moved it in 05 with the addition of mast antenna.
I'm having problem with no radio reception in my 06. Need to check for power at that module.
One of the defrost power tabs was broken off the glass so I'm assuming that hasn't worked for a while either.
_________________________
2006 P71 119k miles, 3.55 traclock

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#4117690 - 08/09/21 05:43 PM Re: P71 Rear Defrost [Re: STR]
Davidzq Offline
Climber

Registered: 01/19/15
Posts: 567
Loc: Seattle, WA
The antenna filter module is located in the passenger C pillar, as is the Remote Keyless Entry/TPMS antenna.

If the light on the switch is on, the grid/module has power (unless the wire is broken/cut)
_________________________
David Hensley
Member of Northwest Panthers Car Club

2008 P71, Norsea Blue SAP
-Message Center cluster retrofit
-Towncar Signature L heated power seats
-Long overhead console
-Compustar remote start/security

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