amplifier connecting
 

Ok I have a boxster S 2000
When I got the car this is what it had.

MB quartz back kit
5.25 MB quartz in doors
Alpine SPS-400 in dash
Kenwood 5120 head unit.

I will Probably put a fiberglass sub enclosure in footwell.

Thinking of powering with Alpine PDX-5

How should I connect back,door and dash speakers on a 5 channel amp?
I could get a 6 channel amp but then would need another amp for sub.
It seems that slim self power subs i saw in these threads are all discontinued
TIA