69 dim dashboard gauge lights

Easy way to find out is pull out the voltmeter and get an actual voltage reading from the wire powering the bulbs, if it doesn't reach close to 12v at max brightness, its likely the rheostat(though it could also be something like a poor ground)...also compare it to what your battery voltage is while running so you have a reference point. It can actually still be the bulbs as well since if they are all the same age, they can slowly age together. A lot of people will upgrade to LED bulbs as well, they too require a rheostat or potentiometer to dim correctly(can't remember which at the moment)