On my 97 grand marquis, when I activate the left turn signal, it flashes like normal, but it also flashes all of the brake lights (left, right, center) as if the emergency flashers were on. When I step on the brake with the turn signal activated, it stops the flashing and the left turn signals (front and back) and the indicator on the dash stay lit. If I step on the brake without the turn signal activated, the same thing happens. The left side turn signals and the dash indicator come on. The problem comes and goes. When going over bumps with the brakes on the turn signal indicator will flicker as if something is loose. Any ideas on where to start looking?

Sounds like shorted wiring. I used to own a full-size Chevy Blazer that did something similar; step on the brake pedal, and the taillights, instrument cluster, and front turn signals would all light up at once. In that case it was trailer hitch wiring that had rubbed against the body and was shorting.

A good place to start is back at your left lamp. Carefully check out the wiring and make sure there isn't any exposed.

The left turn/brake lamp bulb has melted the two poles together, so you're backfeeding the turn signal circuit. check the bulb, maybe put in a new one?
This happened to me and I could clearly see the bulb shorted at the
base.

What he said....

Unless you're running LED's and have a few bulbs backwards..

Still haven't been able to fix this yet. I checked the bulbs and they looked fine. I haven't tried replacing them yet, but, that will be my next step. I checked the wiring in the trunk and everything looks okay. I just took the car on a trip to the beach and noticed something weird. Everything works fine when the car is cool and the outside temp is under 90 degrees. When it's any hotter out, or if I let the car idle and heat up, the problem returns. When in stop and go traffic, the problem occurs. Once I get on the open road, everything starts working again. Any ideas why the heat could be effecting this?