Hi Bill
Welcome to the forum and we few wagoners aka tourers.

FYI: FREE subscribtion to official MB Factory Parts database here:
http://epc.startekinfo.com/epc/welcome.jsp

David Weis in Parts 4 Sale knows of "300TE" in TN that might have seat belts.

Sliding roof aka Sunroof
To best of my knowledge there is no separate relay. It's built into motor located in left rear fender well. Illustration in Section 78 of MB Parts db.

Do a SEARCH on "Sliding roof" the proper name and "sun roof" generic we often use. Also make sure you use info for correct chassis. Ex "124 sun roof"

Your chassis is a 124.092. "124" for basic chassis & "092" for "T" aka "European designation for "Touring" aka "Wagon". The "E" is another chassis id as in 124. MB changed id's so my 1994 wagon only has "e320" without "T" for "Touring" 320 is engine size, yours is 300.

And we can confuse your more with "T" as in "Turbo" but other than "TE" they are diesels.

Please fill in your profile. It helps us know how much or "TMI" aka "Too Much Information" to give you when replying. Good info is your MB's model & yr, your DIY experience and location [helps regarding buying or selling parts].