Took my 2001 mitsubishi eclipse RS 2.4 L in to have an alcohol ignition interlock device installed. Car has had the following problems ever since that it did NOT ever have before: gas gauge doesnt work, wipers dont work, and now car wont start. It has a new battery. Plenty of gas. It cranks just fine but it isn't getting spark. What steps and in what order would a good mechanic take to diagnose and fix the problem(s)?

Put key in the ignition and turn clockwise. LOL, not sure if that is what you wanted could you be more specific?

Your car already has a fuse box diagram located on the inside of the fuse box lid. However, in case its missing from there, follow these two links and you will know everything about fuses:
1. http://mitsubishifuseboxdiagram.blogspot.com/2012/09/fuse-box-mitsubishi-eclipse-diagram.html
2. https://tearstone.com/2000-2002-eclipse-service-manual/

Turn your steering column while turning on the car this should help. I had a problem with mine once and it did this (wouldn't start) but once the battery was replaced problem went away. You could also have a defected battery which also happened to my 2001 Eclipse RS last year.

change the pcm relay or fuse. this relay supply 12 volt to fuel pump and ignition coil. go to the dealer and ask for the diagram for pcm relay and read it carefully or ask a friend to help you. check the fuse that supply 12v to the relay. mechanic shop will charge you for big price for electrical problem. most pcm relay is near the car computer at passenger side.

Grab a cleaning disc it is shaped like a regular cd but it has a row of soft bristles on the bottom side for this exact purpose. another thing I want you to try is use a different brand of writable disc and lower the burning speed. let me know if this fixes this issue.

I have disconnect the battery from my mitsubishi eclipse rs 99 and now the radio ask me for a code I never remove from the car what can I do or how I can get a free code for my car

if i remember right those belts are not together cuz your car has two belts on it one that goes from the crank pullel that runs the a/c and alt and then one thats from the crank to the power steering

Could be an electrical problem. Clean your battery connections. Sounds like you are not getting voltage to the ignition coil. Check fuses. Check if horn, wipers, turn signals are working. Try this test: turn on headlights and then turn ignition key to start. The headlights should dim if the starter solenoid is engaging. You did not mention if it is manual or automatic transmission but most cars have an interlock switch that prevents the car from starting if it is in gear.

I know you don't want to hear this but you will have to have the ignition lock cylinder installed, did you loose the old key?

Throttle Position Sensor or loose throttle cable. Good luck