New radio - no AM reception

[sorry if this got posted twice]

I replaced the factory radio on my 2004 Honda Accord LX with a
Pioneer DEH-5500MP. I used the mounting and wiring kit from
Crutchfield, which is a Metra 99-7862. With this setup I no
longer get AM reception. The antenna is snug and tight in the
socket of the new unit, and FM seems to be fine. The factory
unit had no problem with AM, and the AM on the Pioneer unit
worked fine in my older car. Could it be a grounding issue? If
so, would it just be to connect a wire from the car frame to the
Pioneer frame, or would that create a ground loop and other
problems?

To make manners worse, it appears that once the Metra mount kit
is snapped in, it is very hard to remove.

Other than that, it works great.

 

Years ago, there would be a tiny trimmer, accessible from under the
front escutcheon (take it off) or through a hole in the back, which
is used to resonate the aerial on AM tuned to the 200m end. Just
turn it to get a maxxed out signal. Also, read the FINE manual for
the Pioneer or hget a copy off the 'net.