most modern alternators now provide approx. 14.4v for charging a 12v system. 24v systems run at approx. 28.8v
if you make your circuits to stand 30 volts you should be ok.
Automotive electronics is now based around stinging the consumer for the maximum amount of money possible, you can forget any "commonality of parts", if the manufacturers had their way you would only be able to put their own branded petrol/diesel in their cars. The introduction of...
the SA612 is a neat little mixer ic and includes its own local oscillator, or use an external, i've only used it at lf and hf but the spec states operation to 500mhz
if you do a search for "sa612 pdf" you should find a data sheet with notes on how to use it, also try doing a search...
here is a good webpage, i built a version of this unit and it works well without too much setting up or test gear needed, good explanation too. not sure where you are?http://www4.tpgi.com.au/users/ldbutler/SSBModulator.htm