Need help soldering

[speed demon]

Yes I did shorten the leads and they come straight out of the housing(brushless) I did have brian at work give it a go for me and he has been an elctrian for 20+ years and he thought the wires were either burned up(new motor) or some type of coating. I do use flux paste and probably have plumbers wire. What is a good kind or brand? Is silver solder better? Lead no lead? I solder alot on my old mopar but nothing this critial. thanks for the quick replies.

One last thing if the wires have something on them which is not letting the solder flow could I use some acid(etching stuff) on the wires to get it off?

No pic but I might be able to get one in awhile.