if link fails I apologize
as a standard rule I go solid as neg and spoted or striped as pos
I also test the speakers before mounting (I understand yours is already mounted) so I would suggest when you start on the new head unit take the left or right front door speaker set it on the dash try it on both sides of the dash speaker you can hear the difference more with the low frequencies
The guy I used is in Scottsbluff Nebraska. His name is Gene Cochran, and he works out of his house. He's a crusty old codger, but does some tremendous work. While he had the radio apart, he updated some of the internals and cleaned the heck out of everything. My total bill was $25! His number is 308 632-2520 (I checked with him at the time I collected my stereo, and he was OK with giving his number out)
He also lined me out on the manner in which the old radios were wired (mine at least). The speakers are in series meaning positive from radio to positive on speaker #1, negative from that speaker goes to positive on speaker #2. Negative from Speaker #2 goes to Negative on stereo. Now the stock stereo rocks as loud as the little 51/2 speakers can handle!!!
While it's there you might see if he'll wire in an aux input jack. I had it done with mine, and it allows me to run my I-pod through it. Basically when the I-pod is connected it becomes the antenna source. Cost in parts was under $5. I set mine on a 12" extension so I could just drill a small hole in the dash by the ashtray, which I'm turning into a covert charging / storage station for my phone and I-pod. won't cost much more.