The front speakers in the doors do not have a crossover - separate feeds from the HK amp. Replace the amp and you need to add crossovers.
The rear speakers are "dual voice coil" so they theoretically need two channels per speaker; you could wire the coils in series and drive them that way. Note that the coils are 2 ohm so parallel wiring would bring the impedance down to low for most amps; even 2 ohms is not liked by some less expensive amps.
There is lots of info on the web about the HK system -
here is an entry on my web site for example.
