But if you we're correct, the Automotive Industry would of drop relays from all their vehicles, and they have not.
Never saw a FET at Walmart or a automotive store when you needs a fast replacement
Again, your statement is wrong and based on your simple observation that Walmart doesn't carry replacement semiconductors.
The fact is, that semiconductor devices, including solid state relays / switches, as well as loads of other devices including sensors, etc... are replacing their mechanical counterparts. This because the cost of power semiconductors is rapidly decreasing while their power characteristics are improving, and because of the newer trend of intelligent power systems. In newer cars, intelligent functions and active safety features are required, thus driving the need to use a solid state switch (i.e. MOSFET). With a MOSFET you can measure and control current, easily interface control systems to them (without the need for high current drivers that relays typically need), and also be able to sense and detect broken lights, defective wiring, and other anomalies. You can't do that with a mechanical relay! Also dimming can easily be implemented (i.e. your dash lights, dome lights, door lights, etc...) by using a MOSFET switch and PWM modulation (varying the duty cycle to reduce average power to the light)
The reason you don't see replacement semiconductors etc... is because they are not OEM replacement devices. Because they lack mechanical contacts and moving parts, they are extremely reliable and designed to withstand the entire life of the vehicle.
As well as working as a power engineer for Lockheed Martin, i also run my own consulting business and have done consulting for some of the big automakers for their power systems. And yes, they use large numbers of power semiconductors as switching devices in their automobiles, which includes SCRs, MOSFETS, and IGBTs. Even more so in electric hybrid vehicles. Obviously, relays are not being phased out completely, but there is a push for continuing to utilize more silicon in vehicular systems, especially considering the cost is dropping.
and I will , anyway this would defeat this forums simple question, that 90% of the answers was " Relay "!
And i'm sure this answer is simply based on the fact that its been done this way for almost all eternity, and that most aren't familiar with the design of power electronics.
Again, not saying a relay doesn't work, but rather that solid state is another option.
