Is there a good Dye for Seat Belts?

What year car ? I assume an early A since seatbelts were optional back then.

Look from the underside of the car - you should be able to locate the seat belt mounting holes, they'll be a nut welded to a flat plate of steel about 2 inches square or so. Use an awl and push it through the padding and carpet etc from underneath to locate their position on top. Once located take a hot soldering iron and shove it through from the topside down through the carpet and into the mounting holes, this will burn its way into the holes and thru the carpet/padding and create a heat swadged spot for the bolts to run thru and it wont tear up the carpet. On the outboard holes this will likely be impossible to get the awl thru from underside so try maybe measuring and probing from the topside down using the awl.