how do i get to the wiring harness for the built in amplifier. ford galaxy?

how do i get to the wiring harness for the built in amplifier in my 1999 V-reg ford galaxy? (i have the RDS 2007R radio with 6 CD changer) i need to dissconnect the amp iso lead so i can attatch an after market stereo. i think i have found the amp under the radio+under the compartment tray. but i am unable to get to the cables. do i have to go from the bottom up?