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The brake interlock system is not functioning properly. There will be a little hole to stick a screw driver into, carefully insert it into it to release the handle to shift. You don't need to push hard. You will feel it pushing back since there's a spring in it. The problem could be many things and it should be properly diagnosed by an auto mechanic.
Sounds like one of your door switches are broke,interior lamps were turned on manually at switch, or there is a door still open. Be sure doors are closed and switch is off. If switch is off and doors are closed you can check the owners manual for interior lamp fuse and pull the fuse until you get the problem diagnosed.
Method Detection Limit
Here is a link with location and removal procedure. Good luck http://www.justanswer.com/nissan/16qsn-starter-solenoid-located-96-nissan-altima.html
The noise you hear is the coolant flowing through the heater core. Your coolant is either low or there is air in the cooling system.
My father in law had the same problem on his 2004. He noticeably had a bad camshaft position sensor. You can remove it and check if the what I believe is a magnet has fallen apart. As mentioned before save your money and get it diagnosed before replacing parts. Can't be sure but it's probably not the computer.
Have you checked rockauto.com? As G.T. Service Center said you can get the whole assembly. If you get the whole assembly it will take less time to replace and you won't need a spring compressor.
If you believe the fuel injector is bad you can unplug one connector at a time from each injector. If there is a change in the idle or it runs worse/rough the injector you unplugged is a good injector. Plug each injector back in after checking it. If you have a bad injector the engine will not change idle when unplugged. Of coarse the engine must be on when checking these.
This is a low coolant warning indicator that you describe. Check your coolant level.
Im 99.9% sure there is rust built up between you spindle and speed sensor on the front wheels. You will need to remove the speed sensors and clean the rust off. If you have access to an OBDII computer you can check which sensor is at fault. There is a recall that covers some of these trucks where they will clean off the spindles and replace the sensors if necessary. It is very unlikely you will be able to remove the sensors without breaking them. I myself bought the sensors then performed the repair. Good thing since the sensors feel apart.