The first thing I'd do would be to buy an inexpensive digital voltmeter and measure the charging voltage at the battery terminals with the engine running. The voltage should be around 14 volts. If it is down around 12v, the alternator isn't charging the battery and you will get stranded somewhere.

If the voltage is ok at the battery, it is possible that the connections to the lights are corroded and you are losing power there. Since both lights seem weak, you could clean and tighten the connections at the silver colored relay on the fender near the battery. This is the relay that switches between high and low beam. The connector to the lights may also need cleaning.

On your model, the electrical current for the headlights must pass through the headlight switch on the dash. The contacts and the plastic connector on this switch commonly corrode and you lose power at the switch. The switch will eventually give out and leave you in the dark. I've had this happen on two 240s of your vintage. I was able to find replacements at a pick-and-pull salvage yard but I found many of the cars there had the same problem. Try to get both the switch and the plastic connector in good condition and splice in the connector. If you just replace the switch on a bad connector, the switch won't last long.

The dash lighting in the 240s has never been very bright. I recently bought an '86 Mercedes and the dash is even dimmer than the 240! They must have eyes like owls over in Europe.

If the relay and switch and connections and alternator are good, you can switch the headlights from the old tungsten type to halogen sealed beam units. These are much brighter. Make this change after all the other things have been checked. Good luck, lighting is important.