I have a recently purchased 1978, late year build date, that the interior lights don't work. The headlight switch turned counterclockwise does not turn them on. No power at either of the door switches. If i ground the bulb in each of the three lights, they light. Glove box light works, radio works, and dash lights work and dim with headlight switch. I was told that the timer was replaced and they were working prior to me purchasing. Is there a test procedure for the timer? What else might I check? Thank you!

You may be able to get an idea of what to look for from these (not great!) photos of the Electrical Troubleshooting Manual.

At a quick glance you could check the two fuses mentioned, the CLK LTR CRTSY and the GAUGES fuse, both of which seem to be good for the timer to work.   

Based on and experience I had it will be worth checking that the timer unit is fully plugged in!

You answered your own question. Check for good ground at the door switches. If you can ground the sockets, and the lamps come on, then you have power to them....you must have a ground AT the door switches for the lights to work. The delay timer controls voltage to the sockets, as does the light switch dimmer, but the ground comes from the door switch itself. 😉

Thank you Kentvettuk for the pics, I'll be able to get back to troubleshooting this weekend.  I'll pull the timer box and recheck the connections.

Thank you Adams' Apple, I did run a test ground to the door switches to check that but possible didn't get through the rust. I ordered two new switches and hopefully will get them before the weekend.  If not I'll pull them out to try to clean them.

As you're a relatively new owner of the '78, the Electrical Troubleshooting Manual might be a good purchase😄   There are usually a few decent copies on ebay. 

But in the meantime I came across this on-line version:   https://www.corvettebergen.com/teknisk/1978_corvette_electrical_troubleshooting_manual.pdf

I'm no automotive electrician, but I've found my copy useful for diagnosing a few issues and invaluable when I fitted a new radio and had to unravel "bubba's" work on the radio wiring!😱

Good luck with the car and don't forget, photos are always appreciated!👍

Finally got the electrical service manual and time to work on the problem.  Seems to be a broken ground.  I replaced the badly corroded door switches but no lights still.  If I ground the light bulb itself they come on as the bulb has 12v going to it.  Time to find the grounding point under the very snug dash, should be fun, lol. 

The door switches provide the ground for the lights. The headlight switch does the same...it provides a variable ground. Can you ground the wire(s) AT the door switch to see if the lights work then? The switches screw into the metal part of the body, so they need to be clean on the threads, as well as the female threads on the body. IF the lights still do not work with the wires at the switch(es) grounded, I would suspect the timer module....new or not. 

Thanks Adams' Apple, I just ordered a timer module.  Looks like that might be located behind the glove box. Going to take that apart this weekend and make sure I have one installed, lol. 

I've also looked at my non working horn system while under the dash and the horn relay was missing.  I plugged one in and fuse blew.  Found a shorted horn!  Disconnected that one and now it honks constantly.  Found that the black wire to the horn relay is grounded.  So time to take the column apart and find the short, such fun!

I received the new courtesy light timer circuit and installed, which is behind the instrument cluster on my car.  Same result, lights don't work with the door plungers but they work with the headlight switch turned to the left.  I noticed that the cigar lighter does not work either, don't know if that is related. 

I saw that if you jumper the two white wires at the timer board it bypasses the timer and the door switches work as normal.  I might try that if no other suggestions.  It's a pain as the only way to reach it is to pull the instrument cluster out.  I reinstalled everything to test and to also test newly replaced parts in the steering column for the horn which is now working, yeah.

Hard to believe I have two non working courtesy light timer circuits but stranger things do happen, lol.