Here is a simple fix to a problem that can be important, since proper working of the warning light is needed to pass state inspection.
When I got my 1979 911 SC, the parking brake warning light never came on. Here is the behavior it should have exhibited: when the ignition was switched on, but the engine is not running, it should be on. When engine is started, it should be on if the parking brake is on, and should be off when the parking brake is off. The goal of this light is to prevent people from driving with the parking brake on, which can overheat the rear brakes and lead to loss of brake efficacy.
In my case, the bulb was missing from the warning light. The red plastic panel on the dash pops off with a bit of light prying. The bulb itself pulls straight out, and is easy to replace. There is no need to go under or behind the dash in order to replace the bulb. Easy fix.
There is a switch inside the parking brake that was not working in my case. To fix it, it is necessary to remove the carpet. You will probably need to remove the passenger seat as well.
There is a flathead screw on the passenger side of the parking brake assembly. When it is loosened, the parking brake switch can be moved up and down. If the switch is working, it is adjusted by first setting the brake on two clicks "on", the adjusting the height of the screw until the warning light just turns on. This procedure must be done while the engine is running.
There is a brown wire that sneaks into the back of the parking brake assembly. It connects to the switch, which is really a shaped plastic part with an electrical contact strip on it. Make sure that this wire is connected (as described below).
To fix the switch, or reconnect the wire, the parking brake assembly must be unbolted from the floor. There are three hex bolts that come off easily. There is no need to detatch the heater cables or the parking brake cable. But the long bolt that serves as the pivot for the brake handle and the heater handles will need to be removed, to create some slack. Therr are a bunch of friction disks between the heater handles; be careful not to drop them in the hole in the floor, and note their order for reassembly.
Once there is slack on the assembly, unscrew the flathead screw and remove the switch. If it is not connected to its wire, now is a good time to reattach it. The following pics show the switch after removal, including a pic that shows its part number.
If the switch does not work, regardless of adjustment, the contacts may not work. To fix this, pull the brake handle all of the way back to reveal the copper "elbow" on the underside of the assembly. This copper connector is meant to touch the copper on the adjustable switch. On my car, the elbow had lost its bend, and was not making contact. I bent it with a pair of needle nose pliers. The pic shows the contact after it was bent; it works reliably in this configuration. The pic is showing the underside of the assembly. Bolt it all back together, and it will work.
The following pic shows the friction disks, nut, washer, and heater fan switch unit in the order that they were taken out of the car. If you are missing a friction disc, look inside the hole in the floor; it may have been dropped there during a previous service.
I hope this post is helpful to you. I have benefitted from this forum so much that I wanted to give back. Happy motoring!