I have a 1967 GTO that has a problem with the wiring harness. Some times I have to play with the connection on the firewall to get thing to function (lights, starter etc.) any ideas on how to fix without replacing wiring harness?

Well, the obvious answer is to unbolt it and look to see exactly why it's doing that. I'd disconnect the battery first. It could be corrosion, or it could be loose connectors.

 

That said ... you're talking about issues with a wiring set with half a century under it's belt. A relatively inexpensive and easy to replace wire set. I wouldn't blind myself to just replacing it if what you find warrants it. If you've one problem today .. you'll have another tomorrow. A fresh harness (when necessary) is simply common sense to have a reliable ride. I file this with driving on 20 year old tires. Wiring might not fall under normal consumables for a car, but they definitely do have a best before date.

The fact that you have to fiddle with the wiring harness connection on the firewall tells me that it is probably loose at least and moisture has probably gotten into it  and corrosion may have started at worst. So disconnecting and inspecting it is the logic first step. If replacing it, as Pro suggest (and I agree with him), warrants it it will be a real peace of mind. You should also consider the state of the rest of the wiring in the car. If the wiring under the dash is hacked up or spliced in places, perhaps it is best to completely re-wire the car with a new harness - perhaps with new blade fuses too. A new harness may set you back $500-600 but it sure beats leaving you stranded somewhere or worse, burning your car to the ground.

Since I didn't directly answer your question, you might try tightening the harness connector screws once you are sure you have eliminated or insured there is no moisture getting into the contacts. I believe there are two screws/nuts used to bolt the 2 halfs of the bulk head connectors together between the firewall.