If when you have it on full pass and no low frequency gets through then technically you still have high pass working. If this is the case then tapping into a low pass wire (near impossible) won't help. Two potential problems! (that are likely, it may be neither of these)
1) the amp is broken - do you have warranty??
2) the deck is broken and is not transfering these frequencies to the amp. - my money is on this one!!
Like I said, it may not be either of these. Check them out though cause they are relatively easy to check.