Which iPhone do you have? 3G or the previous gen?
I'm using this nissan ipod integration kit:
http://nissanipod.com/storename/nissanipodcom/dept/242839/ItemDetail-10027321.aspx
It's easy to install, the cables are long, you don't have to tap the SAT receiver in the trunk. It can play, change tracks/playlists, show info etc using radio controls in ALL ipods/iphones. It won't charge with 3G iPhone and the latest generation iPods. The charging issue is due to a change Apple made recently. Starting with iPhone 3G, they changed the charging voltage from 12V to 5V (ie, Firewire charging specs to USB charging specs). These latest gen devices won't charge but it will work fine other ways. There are currently two workarounds:
1) Kensington 4 in 1 charger:
http://us.kensington.com/html/11205.html
This has a female ipod socket at the bottom (to be plugged into your nissan cable) and male on top (plugged to ipod/iphone). It transfers everything except charging features of the stock nissan cable. Charging is handled through the cigarette lighter socket
2) Ridax charge converter:
http://home.swipnet.se/ridax/connector.htm?connipod.htm
This is and adapter similar to Kensington. Female socket below, male above. The only difference is that it has a DC/DC converter so it doesn't need power from lighter socket.
NOW, if you have a custom ashtray dock like me, Kensington will be too jiggly and won't stay in place. If you keep your iphone at the cup holder or somewhere it'll stay flat or static, you won't have any issues with it except the extra cables. Ridax converter is similar as well but I haven't tried it (as it's not out yet completely).
My ashtray dock is similar to this guys:
http://www.infinitifx.org/viewtopic.php?f=5&t=17983
It has a custom PCB and the stock iPod Touch dock cover butchered to fit into the ashtray
edit: if you have the previous gen iphone, just get the front kit and don't worry about these two adapters/chargers...