Multiple devices on a CAN bus via the OBDII diagnostic connector in car
I think this will be safe. My reasoning is this:
- the vendor of the cruise control hopefully made sure not to interfere with the OBD-IDs (AFAIK 0x7DF and 0x7E8, both fairly high ids for an 11-bit identifier).
- the bluetooth module will (hopefully) only use OBD-ids.

If you don't trust the vendors of both devices, you should not connect them to your vehicle even only one at a time. CAN is a very robust protocol, the worst case scenario would be for the bus flooded with collisions for a short period of time until one of the devices involved would stop sending messages. I'm not sure what exactly would happen in this case, but I'm positive that uncontrollable acceleration will not happen: Manufacturers are aware that a failure of the CAN bus can happen any time (just think of a short between both lines).