Basic Car Audio Question: Crossovers/HPF?

[sonick]

Okay, my setup consists of a Clarion HU that does not do have any kind of HPF/LPF by itself. The HU amps two coaxial speakers, and its line out is hooked up to a sub/amp.

I want to somehow get a HPF in there to cut off frequencies below ~80Hz to the speakers (mainly to prevent door rattles), and let the sub handle the lower frequencies.

What's the best way, without adding an amp or getting a new HU, to do this?

What's the cheapest way to do this?

Thanks!

[installerz]

You can use a passive cross over to maintain 80hz & up. Most shops have them in stock. You're probably looking at anywhere from $20.00 and up, depending on the quality.