Volvo Repair: non working break light, volvo 740 gle, brake light switch

Question
Hi Phil-  I have a 1988 Volvo 740 GLE. One of the break lights (passenger side, rear) is not working.  I assumed it was a burt-out bulb however after purchasing a new one and installing it, I discovered that in fact it is not the bulb. It's also not the bulb holder as I've switched the right side holder with the left side holder and it works fine.  Both holders work in the driver side rear and nether holders work in the passenger side rear.

I used a test light to determine if there was in fact power going to the bulb holder. I was able to light the test light on the driver side only.

There is a small wiring harness attached to each side of the rear tail light assembly.  The driver side harness has seven wires one of which (far left red/yellow wire)lights the test light when the break is pressed.  The passenger side harness however has only five wires none of which will light the test light.

Any idea how I can get this bulb to come on?  I'm affraid I'm going to get pulled over for it being out.  All other tail lights work fine.

THanks

Answer
Thanks for being thorough and detailed. If you have checked the harness correctly and checked all fuses, sounds like you may have a problem with the bulb integrity relay. It's the relay that lights up the dash warning bulb. It's the big round cylinder located on your fuse panel. Need to check power going to it and power coming out. At brake light switch check color code and follow to relay. Check for power to both rears by matching color codes.
Wiggle it to make sure it's not just bad connection.
Difficult to remove and access the wiring underneath the panel.
Need to remove lighter and compartment just above it. Need to remove ashtray. Unclip fuse box by depressing both white clips on side. Slide out and flip over, be careful not to short anything out.