The issue is whether your driver's door has a vacuum + air pressure feed in addition to electrical (to switch the pump) or just a mechanical/electrical lock. If it has vac/air/electrical, use the later installation at the pump.
Maybe the factory added the trunk key-activated central locking in 1989 or 1990 - my 560SEL is a 1990.
FastLane lists the driver's and passenger's door vacuum elements on your car (1988 420SEL) as identical (though you have to call for a price quote), whereas my 1983 300SD's differ (because the passenger front has vac/air/electrical, the driver has electrical only). This suggests you have the later system, so do the trunk install. The system will still lock your trunk when you hit the remote, just like the key in the driver's door does.
One quick check which does not involve pulling the driver's door panel would be to disconnect the vac line (the big yellow tube) on the trunk mounted pump and applying 500mBar or so vac for 3 seconds, with all the doors opened. If the driver's door locks (as well as the others and the gas flap) you have the later system, so use the later pictorial on my site - that's the easy one!
Does that make sense? I would follow up with a call to FastLane and verify the locks on both your front doors are identical - I bet they are. Take this precaution or do what I did - trust the factory manual and burn out the pump and the vacuum element as the pump hopelessly searches for vac at the driver's door.