There's a good chance that the external antenna isn't even being used for AM reception - AM on most modern vehicle head units now is so marginalized that the antenna/reception is an afterthought, it seems. It's usually a "bar" type, internal to the unit, for AM.
That said -- on older head units, there used to be a 'trim' adjustment for AM reception. A small slotted potentiometer (or perhaps variable capacitor?) reachable from the outside of the unit. You'd dial in an AM station, then adjust this until you get the best reception. You may need a plastic screwdriver for this. I haven't looked at the newest aftermarket radio that I installed to see if this is still the case or not though.