Nigel, Oznog and Chiba are all very correct and I agree 100% if good regulation is what you want and if you don't mind the price.
Bridge rectifiers (35A) costs around US$1.14 or 57p (Chinese imports). They don't get all that hot when mounted properly and it does not normally feed a load 24/7 in a vehicle.
When you are in the bush with your 4 x 4, you do not always have access to things like LM338 regulators and DC to DC converter spares, but you may find bridge rectifiers almost everywhere in the 3 rd world (where 4 x4 vehicles roam). However, when left with a choice I would also prefer the more high tech solution(s) as was(were) suggested, with the exception of the zener diode idea. Assuming that your equipment takes 6A, the series resistor alone will have to drop 16.4V at that current (44.83 watts!). To stay out of the orange part of the visible spectrum, a 100W resistor is the practical value! Then, at zero load current, the poor zener will have to be tough and may not be commercially available!
Finally, ALL FOUR DIODES in each bridge are used when bridges are wired in SERIES, omitting the AC connections to the bridges; i.e. from +24V side of supply, ENTER at bridge#1 (-), current splits and flows in 2 parallel paths through the bridge and combines again at the (+) terminal of that bridge (4 diodes used) (EXIT). This then feeds bridge#2 (-) and so on....Make a schematic diagram of bridge rect. to verify for yourself.
Sorry for giving you a tough choice!
Good luck!