THERE IS ANOTHER CAUSE TO THIS...

GRRRRRRR......

I have an aftermarket radio. FM reception is OK not great. AM is non-existent unless I can SEE the 50,000 watt tower.

Unless the engine is not running... THen the reception is AWESOME.

Not an alternator whine. Not a spark plug static issue.

Hmmmm.... it gets better.

Engine off. Great reception.
Start engine. Great reception for 2 or 3 seconds, then it dies. Hmmmm...
At idle, turn on the high beams, butt warmers, defroster... BINGO! Great reception!!!
Turn stuff off... dies.
Turn stuff back on? BINGO! Great reception.
Leave stuff on. Rev engine. Dies.
Back to idle - comes back.

What's different about these scenarios? THE VOLTAGE in the car.

Engine off - 12V.
Running idle - 13.5.
Turn on stuff... drops to 13 volts.
Rev engine - 13.8 volts.

AHA!!!

I have a V70. The antenna is above the driver's side rear glass. I bought a new one and tried it. Same. Bummer.

I dug out the stock radio - reception is AWESOME all the time.

Hmmmm....

Measured the voltage at the antenna amp. With my aftermarket radio, it was 12, 13, 13.8 as described above. With the stock radio - it was 10 VOLTS! TEN! Not 12 or 13 like the rest of the devices in the car... TEN VOLTS. Holy voltage regulator, Batman... THE RADIO IS DROPPING THE VOLTAGE TO +/- 10 VOLTS!

What the HECK are they thinking???? Everything ELSE in the car runs on 12 volts.

It appears that the amp drops out if the voltage is too high. It wants 10 volts, and if you feed it 13.5 or more, the amp shuts off.

OMG...

I will tinker with this to try to find the appropriate resistor or maybe a voltage regulator to feed the stupid thing.

So it appears that Volvo specifically engineered a disincentive to using an aftermarket radio.