Here locally, AAA contracts with indy towers. They don't care one way or another -- they're getting paid regardless. You call AAA, and they contact their local tower and dispatch him. I wouldn't make a habit of towing unregistered cars, as I suppose the driver COULD refuse and report you to AAA.

I had the MG towed out to the country for storage, and later that year had it towed home to work on it. AAA warned me that I had used up all my "unlimited annual towing" and that I'd have my membership canceled if I had another vehicle towed that calendar year. I didn't argue the point, as I really should have just rented a tow dolly and dragged it back in myself.

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)