I am not very familiar with the 1991 - 190 but if it's a W 201 than I can be of some help to you because I have a manual in German. If you excuse me for my poor English than I'll translate the R/R procedure for you.
- Put the wiperarm in his most extended position ( pointing to an upper corner of the winscreen ) using the key contact switch ( wiper switch on and than contact on and off : you'll have to be quick for that !). Pull out the key : there is a warning here : leaving the key in and moving the wiperarm by hand could harm the Parctronic - assistent if you have that option. Don't ask me how are why!
- Look at the wiperarm : there is a little plate on the base that you can "flip up" with a smal screwdriver;
- Under that cover a smal bolt with a 4 mm Allen-head. Unscrew that bolt and slide the arm off;
- Left and right of the middle of the big oval cover at the base of the arm are two smal bolts and at extreme left of the wiper mechanism two other smal bolts : take them out;
- Right beneath the wiper shaft is the mechanism hooked to the firewall. Unhook with a screwdriver;
- Take the plug elec. connector from the wiper motor and take the assembly out
If you want or need to, you can further dissassemble by unscrewing the nut on the motor shaft and take the arm of but mark the exact position before!
- arm on elec. motor 5 Nm ( Newtonmeter );
- short lever on motor shaft : 19 Nm (verify the forementioned position !!);
Well : that's it ! I hope it will be of some help to you.
Good luck !