I think the belt system is similar to the M104 on my car. When my tensioner failed it resulted in a loose squealing belt but the car still ran OK. Not sure what would happen at highway speeds though. There is a tensioner shock that provides some tension to keep the car running. Possible that your indie did or did not use an OEM part. Could be a lot of things though, failed belt, broken tensioner, frozen pulley, etc. Dealer will not be cheap but you are relatively close to Tarzana, where there is indie Mr. MB, a well liked shop on these boards.

You can call Enrique and add in another tow and compare prices to what the dealer tells you.

5557 Reseda Blvd # A
Tarzana, CA 91356-2690
(818) 708-8086

glenmore
1991 300CE
2000 C280