Based on your post, you actually have 2 concerns:
1. Power - Normally this is the +12V straight from the battery + terminal. If the existing power wire from the battery to your old amplifier is capable of carrying the combined current (in Amps) of the current + the new, then it is simply running an extension of the B+ line (usually big red wire) from the old amp to the new amp. The negative/ground is simply connecting to the chassis/exposed metal part of the vehicle.
On the other hand, if the old amplifier uses a rather not so big wire to supply it from the battery, you are better running another bigger wire from the same battery terminal to the new amplifier.
You have to check how much current the amp will draw from the battery and select a wire that is capable of carrying that much current without heating up. Offhand a #8 AWG would suffice for most single amp.
2. "Where does the Line cables go?" If you are referring to the Line INputs of your new amp, they maybe connected to the RCA OUTput (line level) of the old amp (it it's got one). Alternately, if the old amp has only INputs and your new amp has both INputs and OUTputs, remove the RCA connection to the old amp and plug them in to the new amp. Thereafter connect another pair of RCA male cables from the output of the new amp to the inputs of the old amp.
If neither of the new or the old amps has an OUTput RCA female, you may need to use a splitter of some sort. Below is an image of a two female RCA to a single RCA male.
You can hook it up in many ways. The simplest would be to remove your existing RCA connection to the old amp, plug in the double female/1male to the INput of your old amp. Connect the cable from the head unit to one of the female RCA. Connect another pair to the other female and the other end to the input of the new amp.
Hope that this be of some help/idea. Pls post back how things turned out or should you need additional information.
Good luck and kind regards.