change your car stereo to one that supports BT
i bought a simple Sony BT enabled car stereo some years back for less than USD200 (they fixed it into my car for me too).
you'd still b able to use it later even if you change to (gasp!) iphone or android phones.
no mechanical parts, no wear & tear, lower audio loss (lower compared to CD quality but almost there), all make BT A2DP audio quite decent.
it also works as my car hands free. no need to wear and charge an additional BT hands free gadget (which we typically really only need while driving), and no clumsy wires too.

as for remote control, can be achieved using BT PAN profile. i once made my n900 connect to my friend's n900 over BT PAN. could ssh, etc. might require remote X if you want GUI.

BT can be kept alive the whole day, not too much batt issues. however, keeping BT PAN connected or BT A2DP music playing will consume a bit of batt (much lower than wifi still i guess).

also, there may b some lag when using PAN & A2DP simultaneously.

all the above achievable if you're willing to mess wth your n900 jst a bit