Whats the secret to getting rid of unwanted electrical noise interference from the vehicle on mobile radios?

Proper grounding, elimination of ground loops, and filters at the battery.

Depends on what is causing the noise. Alternator whine can be eliminated with a DC filter. Ignition noise is more difficult to get rid of. What does the noise sound like? A whine that increases with engine RPM, popping or buzzing that increases with RPM's, constant unchanging noise?

Spark noise, or other sources? If it's from the spark plugs, your wires may be going bad.

If it has to do with anything sound related, then the easiest thing to do is insulate the wiring in question.
This happens a lot when people add powered amplifiers to their vehicle and run the power cable from the engine bay to the trunk. Sometimes people will run the power cable right next to speaker wiring and it'll translate into unwanted noise. The radios are probably picking up electromagnetic interference from a power source.

Otherwise all other suggestions work well but only for those instances.

http://k0bg.com/

All you need to know.

Can you record the interference with an RTL-SDR device?

If you can record the I/Q data, we can probably all put our heads together and figure out what you need to "fix".

ETA:
Also, what band are we talking about?

Also use RG 58 as your power cable. Center to positive, shield to ground. It will shield the power supply from noise. It helped mine a lot.

Depends on the vehicle. Never had an RF grounding issue on a Dodge or GM diesel. Have had alternator whine pop up in a Pontiac sedan and 4.0 equipped Jeeps (some filtering gets most of it out but on my Jeep I'm positive I can hear that distributor spinning at times). Some of the Nissan and Toyota pickups have had not so great grounding in the past but haven't heard a problem with the new stuff.

I'm leaving out one manufacturer on purpose…many RF issues in my experience whether it be grounding, throwing codes/stalling the engine out on AM/SSB, or even turning on the guy's head lights you're sitting in traffic next to with 50W on FM. Maybe I'm just too hard on that brand though.

Uh, for what? 10A would melt that almost instantly. RG58 has a 20AWG center conductor... Did you use RG8 maybe? That has a 10AWG center conductor.

I think this is a horrible idea, it will result in about two of these things:
A) a car fire
B) sorrow
C) both A and B
D) not get rid of the problem
E) mask a noise source in the cabin
F) something that someone who knows more will add

Now that I have your attention, lol, DC Isolation will go a long ways to ridding yourself of DC-induced noise. However, the easiest way to see if there's any DC induced noise is by hooking up an oscilloscope to the leads going to your radio where the radio would be. Drive around and look at the screen occasionally. You should see a flat line with a couple wiggles (being realistic here...) if you see any more than that, you need to run some large fast diodes, along with some capacitors to decouple the lines from AC ripple. Basically, you need two diodes on pos, and neg, then a bank of capacitors (can be small) of different values to neutralize the AC ripple. Most radios have this internally, but they may be "maxed out" already.

OP: Do you have any recordings of the noise? Either I/Q recordings, or cell phone video?

I have some BAD noise in my truck.  It opens squelch on my Baofengs and is ugly on the 857D the one time I tried it off the battery directly.

I will try the scope test.

Can you define 'large fast diodes' a little better?  And give a range of caps to use?

I suspect it is ignition or EFI noise based on the recordings over at the recommended site above.

Thanks!

Here's the noise in my car:
https://www.youtube.com/watch?v=9E8k1jrhjXM (embed isn't working for me)

It's going to be interesting to see what's causing this. I'm almost betting it's Ignition noise, since when the engine auto-stops, it goes away.

ETA:
Large diodes, I mean 30A-ish. By fast, I mean like shottkey diodes, like these here, but with a lower forward voltage drop.

My radio get real bad when I use my phone charger.
I'm just running a 2900. Very clear any other time.