Trying to location part for a 1997 Cadillac De ville lighting control module which is suppose to be under the back seat. Due to no cruise control and no brakes lights. Have everything else that works.

I have a 2003 Deville...battery is under rear seat ...pull seat up and out to raise....battery itself is very easy to change out.
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Did you check that the overhead light is set to "door" instead of "on" or "off". I know it's a simple solution but you didn't mention having checked it.

One is in the trunk on the driver side be hind the rear seat. Second is under the hood on the driver side by the air filter..

Try in the trunk under the speaker where the back seat trim cover is.

check the flasher underneath the dash.

Circuit Breakers A circuit breaker in the light switch protects the headlight circuit. A separate 30 amp breaker mounted on the firewall protects the power window, seat, and power top circuits. Circuit breakers open and close rapidly to protect the circuit if current is excessive. Buzzers, Relays, and Flashers The electrical protection devices are located in the convenience center, which is a swing down unit located under the instrument panel. All units are serviced by plug-in replacements.

Bring it to the Cadillac dealer and prepare for a huge bill. There is something wrong with the suspension system, and they are the only ones who can work on it.

A quick trip to the dealer would confirm but it sounds like your I.A.C. (idle air control motor) may be broken. I don't remember it being too expensive when I replaced it on my 95 caddy de ville. Also check out these symptoms-
Is your car idling properly?
Does it get rough under load or other conditions?
Any idle or off idle irregularities at all?
If so these symptoms may be linked to the I.A.C.
-Dynami

The fuse panel for these controls are located in the trunk on the back of the seat on the driver side under the carpet. then the lid is labeled.

it is normailly the big round thing in the fuse panel on the driver side under the dash by where your feet go