Ok, will fess up here. Yes I am a real ham radio operator of 50 and 100 watt output radios in vehicles for many years. Direct to the battery is always best with a fuse just off the battery terminal, or as Jerry suggested to the power block, which is almost the same thing.
My contacts to Australia off my cigar lighter plugged mobile in a friends vehicle with a mag mount HF antenna still count also, ya know.
My advice is a a tap off of a direct lead to the battery will work best. Further downstream may get interference from other devices in your vehicle on receive. Not eveything in your vehicle is solid 12v dc, lots of computer higher frequency stuff getting into your radio power, and some cases into the case itself. Most likely any transmit interferrence will be off your alternator and need a capacitor. Look for an igniton "condensor" to install directly on the back of your alternator to fix that. I had a GM truck that the "whine" pulsing DC off the alternator got into my radio transmit. A "condencer" (capacitor) from an older Briggs and Stratton lawn mower attached to the 12V output of the alternator under one of the housing bolts resolved that. RF is strange stuff.
Sometimes overthinking can delay the project that could have been completed after trial and error. (as in this case, see what happens, move the power source around to achieve what you need). Another way of saying that is at some point in any project, it becomes time to fire the engineers and start production. If things do not work out as planned, adjust in the field.
Post back of your success or if you need additional help. We all have something inside to want to know what works or opinions of what does not.
