Because of the diverse nature of you failures grounding is not likely. Most of the circuits you mention run on fuses 6 and 7. If the problem came after head removal and you think you put things back there are a few possibilities I would consider.
1. Check for voltage on fuse 6 when the ignition is on (both sides of fuse). The power to fuse 6 may (on my coupe anyway) comes from a hidden fuse located near the coil! Look for a heavy red wire with a black plastic barrel device just below and forward from the coil. If you give the junction a twist you will find a high current fuse inside. This feeds power to fuse 6. Fuse 6 powers brake lights, horn, wipers, reverse lights, and a load shed relay.
2. Many of the circuits are in a cable bundle which penetrates the firewall. Could the bundle have gotten crushed (shorted) during the repair? It is unlikely because the cable runs on the far left beside the brake booster.
3. Main of the dash signals go through a few connectors behind the instrument console. Was anyone up in there?
4. Most likely there are two problems. The turn signals and brake lights are different systems from the instruments but they share a common path with the Hazard switch / flasher relay. There is also little that you could have damaged with a head removal.
When you say your brake lights don't work I assume you mean the main brake lights and not the parking brake light. This circuit is very easy to test out because it is dirt simple. The brake lights are grounded in the trunk. The brake switch supplies power to the lights. At the brake switch above (front) side of the pedal there are two spade connectors. One has two wires; when the ignition is turned on this one should have +12v; if not look at fuse 6 (if no power then see item 1). The other connector with a single wire goes to the brake lights. It should have +12v when the ignition is on and the pedal is pushed. If not (and power was on other connector) then switch is bad.
The instruments (tach, temp, oil) get power from fuse 7. They all also share a common grounding system. All of the signals go through two connectors behind the instruments and from there most go through a cable bundle that penetrates the firewall (instrument sources).
