i made use of the piddle poor headset that came with my MDA compact II, a set of "ipod2 type speakers and a couple of handsfree mics from expansys http://www.expansys....asp?code=128638.
after sitting with a pair of snips and carefully cutting away the plastic from around the volume control on the headset (its glued together) i freed up the contained PCB. removed the wires to the mic/headphones and soldered on a stripped end of the input for the "ipod" speakers (you wiill need to double up the ground for these as the headset has seperate left and right channel ground wires).
the same for the mics after cutting off the plug on the end (the bare wire is ground, the shielded wire is pos).
insulated it all up with copius amounts of tape and then fed all the wires through the car and hid the speakers under the dash. all you see in the car is the 2.5mm plug and a bit of its cable coming out of the dash and the 2 mics which use velcro to attach to your ceiling lining
works fantastic and of course also boosts the system sounds for ringtones, audio and sat nav guidance.
2 x mics @ £4.95 each
"ipod" speakers approx £15 but i got mine free from a sales rep
so even if you buy the headset, max cost of £35 or £30 if you go with just one mic
dont try to run the speakers off the cigar lighter if you have the MDA comapct II on charge from it at the same time, you do get a power buzz feedback, at the mo, im running the speakers off off 4 rechargable AAA's but am going to wire in a separate cigar lighter to the ignition power supply to overcome this problem (and have them power down with the ignition)