If you're going to replace the relays with solid-state ones, screw the load, it won't care =).
If you choose to do that, you won't need a resistor at all. First of all, find the LEDs you want to use. I'd say go for at least 8cd. They'll have a voltage drop, usually between 2 and 3.6V. Lets just say its 2V, to keep it simple. If you put 6 of these in series, the voltage drop will be 12V, thus they'll only draw the current they want, almost like they were resistive loads. (If you hook a 2V LED to a 2V lead-acid cell, instead of shorting, it will just draw the 30mA or whatever). So wire them up in for sections of 6 LEDs (but arrange them in 3 rows of 8 ) and you won't be wasting any power at all, and you won't need to worry about hot resistors. If you want some of them always on, just wire them up to whatever controls the front/rear lights I suppose.