Hi there,
I am in the exact same situation as you. Installed a Kenwood headunit last weekend in my 2007 transit connect and a pair of spekers.
I have the same colour speaker wires and have also used white as positive but I am not 100% sure this is correct, did you confirm this?? (you can buy a speaker adaptor lead so you do not need to cut the existing one but mine came wrong and I could't wait)
As for the steering wheel interface i used this adaptor , plus an extra lead specific to your head unit's make, works like a charm!
Any help on the speaker wire colours would be much appreciated (white and light blue at the speaker end)