Hey Fam.... Im new to the forum. My 2004 Xtype 3.0 AWD wouldn't start because initially the battery was dead. So I went out and bought a brand new battery installed it, and find out the starter was bad. Install a new starter in car will still not start. Any help would do, plus I heard of something call an immobilizer?

 

Use the search function at the top of this forum

 

It would be a good idea to diagnose battery and starter before replacing them. It is not likely both of them died at the same time.
Do you hear click when you try to start it or not? If you hear click, try to turn crank with wrench.
My S-type lost key identification function while I was visiting parents 700 miles away.

 

The piece or switch you are searching for is called "Inertia Switch", it is located on the passenger side of the vehicle in the passenger front footwell. Look for a small plastic cover similar to the tow hook cover on the front bumper. Approximately where the passengers right knee would be. Remove it and press the switch to reset.
It is doubtful that the switch is your problem because it should not stop the engine from turning over. Its purpose is to shut off the fuel in a collision. The engine will still turn over but will not start (no fuel).
Time now to get back to the basics in diagnosing your "no start" condition.
First before I can make recommendations I need to know when the vehicle was built. Inside the drivers door rear post area should be a label that has the build date. The 2004 models have 2 entirely different electrical systems depending on their build date. Model 2004 from Jan to Mar 31 have one harness and April thru Dec have a different one.
Knowing the build date I can correctly identify the relay locations and fuses associated with the start sequence.