[pgarner]

radios generally have their own connections to the head unit but at the other end its usually an ISO connections 2 big block of 8 connections ( not all are used ) 1 for speakers and 1 for power

unless your car is old or something stupid chances are that it will have an ISO connector as well so they just clip togeather. if it doesnt audioleads do wiring harnesses / adaptors which clip into the cars wiring and to the ISO of the radios. these normally cost around a tenner. other option is work out what every wire does and use connecting blocks ( this can be hard)

on your new head unit you will have 2 wires on the power cable 1 is red and 1 is yellow these are battery and ignition switch both should have some sort of connetion on them normally bullet type.

after youve fitted you head unit before you squeeze it into the space switch your ignition on and make sure the unit comes on, then program a channel into the radio and store it,

then switch your ignition off and remove the key, give it around 30 secs then switch it back on

if the head unit didnot switch off when you turnd off the key or it has not stored the radio channel then these 2 wires will need to be turned around

as for a guide when you get a head unit it will have fitting guide with it
hope it helps