positive earth clock
Looks promising, but I haven't read the specifications fully. It claims to convert 12v to 12v, with isolation from input to output.

Connect the inputs to the 12v supply from your car. Connect the +ve output to earth, the negative output to the supply input of your clock.

Note that it claims to be 78% efficient, which is presumably measured at 1 watt, which suggests it could consume up to 280mW for itself at that power, probably a bit less at lower output power. This places an extra load on your battery of up to 20mA, or up to 3Ah battery capacity per week. If that extra load is acceptable, then you should be good.