There is usually a gromit in the bulkhead where you can run the power through. not sure where though. If there isnt, drill your own. make sure you seal it properly if you do.
running power cables under the strips is fine though you should seperate power cables from RCA leads as this can cause interferance.
if you have a beasty system you should tap your own earth point to the cars body, otherwise there is usually one near the rear lights you could share. If you do make your own earth make sure you give the area a good sand down to make a good contact.
Hope that helps
