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I have an issue with my Reverse Lights. I have a good fuse, and known good
lamps.

I pulled the lamps and found that I have 12v at the sockets. The circuit
design is a bit odd, the reverse switch feeds the left side lamp assembly,
then jumpers out on another pin over to the right side lamp assembly. I get
voltage at both lamp sockets when the lamps are both removed. But, when I
insert either lamp, the voltage drops to 0.

Voltage applied is a function of the load on the circuit, so I have decided
that I have a very dirty reverse switch on the transmission -- it's a manual
trans, by the way. The switch passes 12v, but has very high resistance,
which lowers the current available in the circuit. When the bulbs are
inserted, the load exceeds the available current, and the voltage drops to
0.

The tail lamp assemblies share a common ground, and none of the other lamps
are giving me trouble, and I can connect the reverse lamps directly to the
wires that give them 12v and ground, removing the bad ground scenario from
the equation, and the lights do not come on. I can plug the lamps into other
sockets, and they work fine.

If I connect my volt meter properly, it can tell me the current that is
available -- I haven't done this test yet. I would expect to see something
in the range of 10 amps (this is the rating of the fuse) to make the lamps
come on. If I only see 0.5 amps, this would seem to support my theory of a
bad switch. Is that right? Any other ideas?