Before starting the car I check the voltage of the battery and it sits at 12.8
The car at cold start it charge at 14.3-14.4 without turning any headlights or radio.
Once, I put load , by turning on the heater, Headlights. The volatge starts going down to 12.3-12.6 voltage.
After, everything is turn off, the voltage slowly goes back to 14.3-14.4
Could it be a bad voltage regulator inside the Aftermarket alternator or something else electrical.
Basically when you place a load on the electrical system as you did...the alternator output is just at the break even point. It should be able to charge higher into the 14 V area. Its possoble the internal regulaor is not adjusting or the alternator is maxed out output wise. In that event the alternator should have a load test done off the car to see what it can truly output. But it is a good idea to first check the battery in itself that it is good condition. many alternators have been replaced because of the battery being marginal. I myself included...
The battery is brand new, the alternator was a oreily replacement. I tested the alternator at store, and came out good.
Should I get a alternator from Toyota or should I replace the Voltage Regulator on the oreily alternator.
The Toy alternator is going to be expensive no doubt...I have a 1996 tacoma and those dealer parts are high..LOL! But in this case you already tried the O'Ro\illey Alt....Maybe its Toy's time to purchase their alternator as you already tried the OTHER. As a last ditch before purchasing the Toy Alternator replacing the regulator from O'rilley would either fix the situation or remain the same....Your call.....