Installing amp  

Thanks! But I am using the amp to power a sub-speaker. Any ideas?

If by sub-speaker you mean a bass (or sub-bass) then you should almost certainly use a filter + attenuator/impedance matching system between the deck output and the amp. You would certainly need to filter the output to give preference to bass frequencies and, if your feed to the amp is derived from the speaker output of the deck, you may well need to attenuate the filtered output to match the input characteristics of the amp (which you should be able to get from the manuals I linked to if you don't already know them).

Such a filter + attenuator/impedance matching is something that most enthusiasts would put together themselves. I'm afraid that, although I know enough of the theory to understand some of the requirements, I'm not able to tell you exactly how to do it. What I do know is that, if the only output from the deck is designed to drive speakers, then the impedance is probably rather too low (in the range of 8 - 16 ohms where about 400 ohms is normal for a line input) and the peak voltage will almost certainly be too high for the amp.

For best results you will probably need a small active (powered) device to split the output between your main speakers and (via the amp) your sub(s). It's worth noting that stereo output to sub-bass speakers is a little pointless. The human ear doesn't distinguish 'position' in a sound source below the mid-range, so a central bass speaker (or two fed from the same souces) is all you actually need.