How can I drive a 12V automotive relay with a uC running on 5V
Higher-power transistors generally have lower gain, so you almost always end up using a small transistor (2N3904/3906) to drive the big bugger, which in turn drives the relay. For automotive though it is easy to find relays with less than 100mA of coil current (easy to drive) but with contact ratings you'll be looking for to drive the A/C clutch.

But transistors to switch big current are passé; why not use a FET? Logic level FETs are available and capable of switching dozens if not hundreds of Amps at the voltages you're looking for, and appropriately sized would need a modest (if any) heatsink. What are the current requirements for your A/C clutch?

I disagree about needing an opto; your micro is already sharing the same common as the rest of the vehicle and the inductive kickback would be handled by flyback diode in the FET or with the diode you place across the relay coil; it's highly unlikely that a failure mode would present 12V at sufficient current to damage an I/O pin. If you were nervous enough you could always use a transistor to switch the FET.