Hey guy!
We are working on the same thing!!!!

I am giong to convert my 700M to a car pc for my Lexus IS300 too!
But i am still waiting my M2-ATX ITPS and Touch monitor.
Hope i could help you.
1,
For the PW button wiring:
The Top 2 points of the button are the same thing +5v.
The Bottom 2 points are the same GND.
So you should use only one of each Top or Bottom pionts.
But for me, I would use the +5v in the back . There's a hole at the bottom left conner of the PW button, Just see through this hole, tear the protecting plastic in the back of the motherboard, you will find a piont can be use for soldering. And there will be more easy to soldering from there.
I think that is ungly to soldering from the front too. Especially sometimes when you have to take off the pw button panel.
2,
If you want to make the external PW button to be unplugable. Get use of the moden RJ11 port.(If you think you are using the modem , then just forget it.....)
Just unplug the moden wire from the laptop inside , then connect the PW button wires to the moden's inside-plug. Now use a long Telephone Cord for the external PW button from the oudside RJ11 port.
I could up load some pics if you want.
Have fun.
dcz838