power antenna and FM question

[r4daytona]

My girlfriend has a stunning 63 valiant convert.  When we got it we were told the radio was converted to FM ... there is a power antenna in the trunk.  It worked for the most part, but sometimes the station would or would not come in.  Last summer it just stopped working and the antenna wouldn't even go up.  Mike at Retro Radio (they did the radio in my daytona)told me that if the antenna isn't receiving/getting a good signal that the radio will go in mute mode.

So I went to a automotive radio shop and bought this power antenna that you can hide under your dash.  It looks like a rectangluar block and has one wire to hook power to it.  So I plugged it in, hooked it to power and the radio worked .... but only received AM.  The power antenna has what looks like a coax cable going into the radio via an adaptor.  This new one just plugged in to the radio.

So do i assume that there is something in the power antenna that boosts the signal to receive FM???   And then just get another power antenna?