Hmmm... that's weird! Do you have E-code Hella H4 lamps, or DOT? That makes a big difference. Assuming you have E-code lights, the next thing to check is voltage at the bulb... it should be within 1 volt of the measured voltage at the battery terminals. Relays will helpa LOT, as mentioned in this tech article (notice the percent light output lost with a tiny voltage drop) :
http://lighting.mbz.org/tech/relays/
About the dash instrument cluster lighting, well, the design is just poor from the factory. I've been told that cleaning or replacing the dimmer rotary pot will help. You could try 5 watt bulbs (stock is 3w) but beware, they can melt the dash if left on max for very long. Wouldn't hurt to check your alternator output voltage on all 3 cars, maybe clean up the battery terminals & ground points, and perhaps clean & replace all the fuses...
HTH,