Brake lights-intermittent problem

On my 99' GCL, I've been trying to solve my iffy brake lights.

Fuses are all checked out, and every bulb looked at and shaken and looked at again. Top third eye light works okay in every situation.
Nothing has been done underneath the engine or lights, no electrical work or repairs done.

Last thing, I need now is for some cop to stop me for broken taillight, when I can hopefully resolve it before they see it.

Here's the problem: (the concern is when brakes are pressed)

Pressed
Engine: Headlights: Brakes: LEFT tail: RIGHT tail:

OFF OFF NO OUT OUT
OFF ON NO OK OUT
OFF ON YES OUT OK


ON OFF NO OUT OUT
ON ON NO OK OK
ON ON YES OUT OK

EDIT: opps, it looks okay during the typing part, but the matrix got bunched up, you'll need to re-match them if needed, sorry.

Bad ground.

Where do I begin to search? Do I use one of those voltmeters (never used one before)?

No, just remove/clean all the grounds on the tail harness. The 99 is a different body than mine, so I can't give you locations.

I had the same issue with the brake lights and replaced all of the sockets and that solved the prob. if you look at you sockets you may notice they are burnt looking- I got new ones from the dealer and replaced them myself- peice of cake and it saved be almost $200 bucks to not let the yahoos at the dealership run a diagnostic and charge me an arm and a leg.

good luck!!