Actually, most of the European automakers (Mercedes included) use vat-
or drum-dyed leather, where the dye is soaked all the way through.
It's mostly the American and Japanese companies that spray-dye their
leather.

Regardless, MB-Tex is vinyl, not leather, so any commercial vinyl dye
will work just fine on it, as long as it's properly cleaned first.  

Here's a link to a place that has the  MB-Tex Grey #138 vinyl dye in
aerosol cans.  That should be the appropriate grey for your 1992
model.  If not, they'll custom-match whatever vinyl colour you have.

http://www.worlduph.com/semdye.htm

Hope this helps,