[Hanuman]

Model year is going to help when it comes to electrical issues. There are many variations, along with some features that were added along the way.

The inputs to the voice synthesizer/warning module are fairly basic and rudimentary.

What you are now describing is a load issue. The greater the load on the supply, the worse it gets. Old wiring, tarnished connections, and internal problems affecting the relay contacts. All sources of increased circuit resistance.
quattro headlights (poor output) have been the source of many complaint over the years.

Anything less than 12v at the bulb is going to have a rapidly decreasing output. (Edit to add- frequently, it’s really low, in single digits, 8~9volts)
Each bulb is drawing several amps, which will also manifest as heat in other areas, like the headlight stalk, and the fuse/relay box.

There were several ‘high power’ wiring looms offered over the years, which used the original circuitry to simply switch an additional relay to each headlight, and provide a different route for the headlamp supply.
I’ve seen a few, and they were a fire hazard due to really crappy quality. But the concept is sound.