The symptoms indicate a reception problem rather than an interference problem. Since nobody has checked your antenna, that would be the component to check next. I checked with all of my instructors, and nobody can recall Acura using an amplified antenna, which would cause similar problems that normally plague Volkswagen owners.
Changing the head unit is the action that appears to have caused the loss of AM reception. Two usual causes are worth investigating: either the head unit has a defective AM section or front end, or the antenna has a problem.
The head unit is possible; however quite unlikely. To prove this, plug a different antenna into the head unit and see if the AM changes. This antenna does not need to be mounted since it will still show some level of change if the Acura antenna was faulty. You should find that AM seems much more responsive to antenna position, although not perfect.
We finally narrow it down to the antenna. One idiosyncrasy of a basic automotive antenna is that it will allow FM to function nearly perfectly even though the cable is broken. AM, however, appears very weak and saturated in static. Starting the engine produces a dramatic increase of engine and system noise into the tuner. This characteristic was known to lead many experienced installers to the conclusion that the damaged antenna cable was not the real source of the problem, because of the good FM performance. Based on your experience, I suspect the installer may have drilled through a third cable...
Q I own a Land Rover with a 24-volt electric system. The two 12-volt batteries are charged by a 90-amp alternator. I would like to add a number of 12-volt accessories (radio, exterior and interior lighting, etc). The max load of these accessories would be about 400 watts. Which converter from 24v to 12v do I need to make sure I can have this load? I thought you might have someone working there that would be familiar with this information. I look forward to hearing from you John Carino
Although I've never worked on a 24 volt Land Rover, I have dealt with the Toyota Land Cruiser, which is a 24-volt vehicle as well. I suspect there was some warm British beer being consumed with Japanese sake when both vehicles were conceived.
I've encountered several instances where an installer thought he could tap in between the batteries. This does measure 12 volts at that point, but drawing current from that spot really messes up the charging system.
Using a DC-to-DC converting power supply (often called an "inverter") works reasonably well as long as the right capacity is chosen. For 600 watts at the high side, you would need an inverter that can deliver a constant 50 amps at 12 volts. Whether you select a pure sine or modified sine wave unit is up to you, but pure sine is likely to reduce the possibility of 60Hz AC ripple from causing noise in your audio gear.
Electrocraft sells a 5-amp model that would be useless to you for about $100 in the U.S. The biggest model they have ever made was 30 amperes at a price of $400 U.S. With the noise problems and other difficulties factored in, I always fall back to the "old faithful" method. There should be ample room under the bonnet (hood on an American car). Buy a surplus 12-volt alternator, a regulator and a small 12-volt battery and have a mechanic fabricate a bracket to hold the second alternator to the engine. You have now produced a dual voltage electrical system that allows you to load up 12-volt equipment without ever risking damage to the stock 24-volt system.