An active crossover goes on the low level lines (RCAs), between the head unit (empeg) and the amplifier, it also has active components inside that need a power feed (not very much current though). The passive crossover goes on the speaker wires between amp and speaker and consists only of passive components (coils, caps and resistors)

The very simplest thing would be to put a cap in series on the speaker wires to the normal speakers (aka "Bass blocker"). That'll provide you with the least possible filter slope (6 dB per octave) - higher slope is normally considered "better" - ie if the filter frequency is specified at say 200 Hz, you will still have signals lower than that pass, but the 100 Hz signal (one octave lower than 200 Hz) will be attenuated with 6 dB, the 50 Hz signals attenuated 12dB etc.

To prevent the sub from playing higher than wanted frequencies you can add a coil in series on the speaker wire - this'll provide a 6dB low pass filter.

It is easier to build active filters with higher slopes than passive ones.

How are you planning to power your sub? Connecting it to your existing amplifier in "tri-mode"? (ie bridged over the rear channels while keeping the current rear speakers also) In that case you need passive filter components in order not to bring the Ohm load down too low on the rear channels. (Also, if you were to filter already on the RCA signal, you would not have any high frequency content remaining to be amplified and sent to the rear speakers.)

Here are some links to more reading:
Crossover slope
Electronic/Active crossover
Capacitor & Speaker
Inductor & Speaker
Passive crossovers
TriMode speaker connection
TriMode power

The entire site is very good for understanding basic car [audio] electrics.



/Michael