Take a look at the maintenance schedule in your owners manual. There are a lot of maintenance items recommended to be replaced by 100k miles. Don't skimp on ATF changes. Watch for oil leaks from the many cooler lines. Use high quality oil filter.
Whats the engine trouble code? Do you know what's wrong with the wiring? Is the car charging at 14v?
It's unfortunate how often these cars get butchered with aftermarket electrical installs and removal.
2006 P71 119k miles, 3.55 traclock