What's the relay next to the fuel pump relay?
 

On a 1991 W126 firewall? I just replaced the fuel pump relay and I'm thinking that I should also replace whatever that other one is to preempt any problems.

The one next to it is marked 'Klima' and controls the air conditioning/ventilation in your car. There's no indication on this board that this is a high failure rate part, so I would suggest if you a/c works well that you save your money.

If the relay fails, your car will still run to the local dealer where you can get ripped off for a new one - or buy from FastLane and wait a few days.

Thanks TP:
Is that the relay for the blower function or for the cooling function? My A/C only blows cool at highway speed. Maybe that's a reason?

Both
 

You may have a blown mono-valve - it sits in the little cube to the left of the two relays you mention under an alloy cover retained by four screws with an electrical connector on top. More or less in line with the passenger's seat.

Check this forum using the search function - an easy DIY replacement and about $30. With the engine cold take the top off and pull out the valve - a slender stick-like thing. Check if the rubber diaphragm on it is torn. Is so, replace - ~$30. If it's OK, suspect your relay.