Poor AM reception after changing stereo head deck
Alfa 156 2000 twinspark year 2000. I had the same problem, virtually no AM reception after removing the factory Clarion head deck and replacing it with a Panasonic but I have solved the problem. Bear in mind this worked on my car and its at the respective owners risk to carry this out with no guarantees but anyway....
I was able to buy an aftermarket loom that connected to the Alfa loom and although it didn't perfectly match the radio it meant I could retain the Alfa's original terminal connectors
On the factory wiring loom on the white plug there is a Red/green wire. (The others on that plug are earth (black), then 12v constant battery power, 12v ignition feed when key is on, 12v when head lights are on to dim or power up display on the factory unit).
Check the red/green wire with a test light. This one should have no power at all, key on or off.
Connect the blue wire that is labled "Aerial" or "mast" or similar as per wiring diagram from the new stereo head deck wiring loom to the red/green wire on the Alfa loom. (Most aftermarket stereos appear to have this wire in blue to activate an electric aerial). Short of having the correct spade terminal to insert into the wiring block, it will require a simple piggy back type connecting terminal, that is the type where the wires a side by side and the terminal clamped on to them.
This then supplies power to what I have seen described in other replies as an AM booster that is standard on the car and you should now have crystal clear AM.