Not under the car.  It is under the dash.

The speedo cable is thick and hard like the coaxial cable used in TV
signal.  It goes directly into the back center of the cluster (going
thru firewall above the brake pedal).  The brass hand-tightened 'nut' is
about 2cm in diameter.  If the dash layout is the same as my 84 190D,
you need to remove the air duct segment that goes to the left vent.

I am not sure if 88 has changed to use electrical oil pressure signal.
If not, the nut (10mm) to secure the oil line needs to be loosened too.
   This can be done from top after the cluster is pushed out.

Since you are doing this from under the dash, if you do not have the
hooks to pull the cluster out, you can push it out from below.