It's probably down to voltage drop.
By the time the 12volts from the battery has travelled through 2 or 3 metres of wire, about 7 or 8 crimped on spade connectors, two switches, a fusebox and a relay, all 20 plus years old it could well be down to 9 or 10 volts.
The fusebox is normally the prime culprit, the fuse ends corrode against the holding springs which causes a less than ideal connection, and voltage drop. When in use, feel the fuses (both ends) to see if they're getting hot. If the fuses are warm or hot to the touch, there's a bad connection there. Fit new fuses, clean up any corrosion with fine sandpaper, make sure the fuses are being gripped tightly, tweak the holders if nessacary, then give it a quick WD40 - ing afterwards.
Also worth checking all the other spade connectors to see if they're getting warm. All spade connectors should be a nice tight fit onto a clean terminal with no corrosion.
Finally check the headlights are earthed to the bodywork correctly, and not just via the metal to metal contact of light to rim, and rim to wing, and wing to car. Follow the earth wire from the back of the bulb, and make sure it's got a good clean corrosion free connection to the main body of the car.
Dull reflectors behind the bulb can make the light output seem very poor.
If you pull the reflector from behind the headlamp glass, and look into it, it should be like looking in a (curved) mirror. You should see a good sharp reflection of yourself. If it looks like you're looking into tinfoil (even the shiny side) and you get a fuzzy, dulled, or no reflection, then you need new headlamp reflectors, or to have yours re-silvered.
Dave.