Most common fault on TC system is the breaking up of the gears on the unit mounted on the bulkhead ( high up drivers side). There is a small inspection flap on the unit to enable you to look at the gears.
If you do discover the fault is the broken gearing then before you pay an arm and a leg for one, it is identical to the early BMW CRUISE control unit on the E39 such as this ( you just have to swap the cable)
https://www.ebay.co.uk/itm/Original-...TL6c:rk:6:pf:0