Hey, this may be a dumb question, but I've always been curious: couldn't you just use heater hose that was the right diameter to replace the NPI heater tube? It always seemed like it would be an easier option that was just as effective as popping in the PI heater tube. I could be wrong, but if it would make this job easier I'm sure all of us NPI folks would appreciate it!
1999 Crown Victoria LX - 69,420 miles (nice) as of 1/27/21, Toreador red, 18x8 Anthracite Torq Thrust M's, AEM Dryflow panel filter, custom 2.25" dual exhaust with H-pipe, Magnaflow XL mufflers, Jones 22" long 4" slash cut tips, Kenwood KMM-BT225U head unit, PPI A600/4 4-channel amplifier, PPI i650.1 monoblock amplifier, Pioneer 6x8 2-way speakers, two 12" JBL subs in sealed enclosures, 2/2019 to present
2003 Buick Regal GS Joseph Abboud edition - as close to a Grand National as I'll probably get, traded in 2/2019
1996 Lincoln Town Car Signature - 1st car ever bought, dubbed "The Emancipator" by friends, rear ended and totalled in 5/2016
2002 Buick Rendezvous CXL - passed to me by the folks, handed down to younger brother in 2/2015