For older cars such as a 1967 VW Beetle one of the best places to find parts is a junkyard. Wire harnesses, engines, and body panel are all readily available in local junkyards. Classifieds websites are also good places to find deals for these cars.
If Available you buy the correct Wiring Harness.
you buy one pre made from painless wiring
when you buy the harness to wire in, the intructions for correct wiring will be on the harness packaging
Buy the Haynes manual for the car. It has a wiring schematic that can guide you. If a complete re-wire is required by a pre-made wire harness, many VW restorers caryy them , try www.cip1.com they may carry it. Then follow the harness directions.
What you need is an alternator wiring pigtail harness from the dealer!
You can buy a speaker harness. An adapter that connect the positive and negative from the speaker into the stock harness.
A john Deere Dealer. The wiring harness comes in 3 different pieces. The last time I checked the three totaled about $1,000.00.
You should never have to replace the wiring harness, and if you do get a professional to do it. Now if you are talking about the wiring harness for a after market head unit, then you just buy an adapter. AND ALWAYS DISCONNECT THE NEGATIVE BATTERY CABLE WHEN MESSING WITH YOUR CARS ELECTRICAL SYSTEM.
You need to buy an adapter harness. Do not cut into your wiring harness.
A salvage/junk yard.
The best thing to do is to buy a wiring harness for your car and new stereo. These are readily available on eBay from $15. Just search for 'stereo wiring harness'.
Dealership, junk yard, Ebay, etc.