T-taps are the worst type of connection you can make, especially in a vehicle. They're probably the easiest, but worst in terms of reliability. The constant vibration encountered in an environment such as a vehicle can cause the t-tap to work itself loose after time. To add to that, the electrical connection between the wire and t-tap, then the t-tap and spade terminal is poor, causing resistance, sometimes enough to cause a large enough voltage drop to keep the circuit from operating properly. The best way is to strip the insulation back from the target wire, wrap the new wire around it, then solder. I can't tell you the number of times I diagnose problems with aftermarket equipment and find out that it was caused by t-taps, whether installed poorly or even correctly and then just coming loose over time. Probably about 80-90% of the troubleshooting I do is from poor electrical connections caused by installers using t-taps.