87 5ksQ Reverse lights don't work

[Drew]

Hi list;
I have an 87 5000S Quattro that failed Mass state inspection for no reverse
lights.  I have checked fuses and bulbs, they are good.  I put the car in
reverse and checked for voltage on the leads at the bulbs, no juice.  I am
thinking that it may be a reverse light switch, but do not yet have my
manual, should arrive this week.  
Can anyone tell me where to look for that switch?  Any other tips on getting
the lights to illuminate?

TIA 
Drew 

A. Drew Bruning
Barre, MA