My Basslink II included coupling capacitors, I think intructions said use if humms... One in series with + of speaker input (you are using speaker input?) I used phono inputs and line level convertor at OEM radio .Check Infinity site for pdf of instuctions. I could check the value of the caps if you like? Just bi-polar caps , like some use as "bass blockers". The odd thing is, speaker act as microphone and (with all else off) and feedback builts on itself, hence the "closed truck = louder. Just my 2 cents Mark