Elec. problems 74 Dart

Good grounds for the stop lights? There should be no relays involved. All that wiring usually runs inside the car too, so less chance of issues like under the hood. I'd trace back from the stop light bulb sockets and find the grounding mechanism. Some ground right to the metal housing, others have a separate ground wire. Also, use a volt/ohm meter to see if electricity is making it thru the wiring all the way to the bulb when applying the brakes. Lastly, check for a broken wire/open circuit between the switch and the bulbs - simple continuity test from the switch to the bulb.
Temp gauge - sending unit is easy to replace, it's on the motor, purple wire slips over a threaded stud on the sending unit. Purple wire goes thru wiring harness to firewall to a connecter leading to the gauge or a pin on a pc board. Check for continuity from sending unit to endpoint of harness. Gauge could be bad, or printed circuit board that gauge connects to could be corroded under the sheet metal nuts that hold the gauge to the pc board. Scotch brite/400 sandpaper to lightly clean copper contacts. Replacement gauge thru parts wanted here on this board.