78LT w/AC, 350cu., it's located between power booster and wiper motor on the firewall. one wire comes out of the fire wall to one side of it, the other side goes behind the dist. on the fire wall feeding the hood light and A/C unit. Think it's a relay or buss fuse? What is it? How to check if good?

could be a fuse in a tube style holder, if so it will twist open with a long and short end. the tube may break if opened.it can be tested with a test light(instead of opening). first, find where the wire leads/ends towards a/c
1) ground the test light
2) turn ignition on
3) test for power
if you have power the fuse is fine. also, you can't damage anything by using a test light.
oh, if it feeds the hood light and the hood light works, that fuse is fine.
good luck

What color are the wires attached to it? This diagram in the lower center shows a 25 amp fuse for air conditioning. That might be it if it has an orange wire on one end, and a brown on the other end.
http://www.nastyz28.com/camaro/wire/78w-acopt.jpg
If it is a fuse, it should read basically zero ohms with an ohmmeter.

Also, the tach filter is mounted on the firewall, but it has two brown wires on it and it doesn't attach to the air conditioning or the underhood light.

circuit breaker for the a/c

One wire orange the other brown. A/C relay probably, thanks. I don't use the A/C so couldn't tell.