Resiatnce of headunits

[coollg]

How can i find out/work out the resistance of the ouputs on a headunit

i have currenty got a Pioneer DEH-4700mp

and i will be rebuilding my ice up the back with instead of one sub in my costune desing i woudl have 2 along with tweeters which run though a crossover so i need to know the reistance so not to fook ma headunit up

regards lee