I just put in an after market jvc radio and I started the car and then I shut the car off and then I couldn't start the car. It would even crank and there was no power to the radio eather. I unpluged the radio and try to jumpstart the car but nothing.

Question edited for typos.

These are the D21 wiring details. The best way is to get a conversion harness D21 to JVS from Crutchfield.

1994 Nissan Hardbody Pickup Stereo Wiring - The12Volt

https://www.the12volt.com > installbay > stereodetail
1994 Nissan Hardbody Pickup car stereo wire colors, functions, and locations.

Nissan Hardbody radio wiring colors

https://forums.nicoclub.com > ... > Nissan Trucks Forum
Mar 5, 2011 - have rear wires i ran my own but this was schematic for after market plug to harness so colors should be right unless back are missing and ..

Could be a weak or failing alternator. Have it tested.

Could be crankshaft sensor.

fuse is in the fuse box under the dash but to be honest the MK1 or first generation focus had radio problems. The easiest fix is to replace the radio with an aftermarket one.

either remove the battery terminals each stopping point
until you check starter and ignition and alternator

if the parasitic drain started straight after the radio install
return to that after market installer
and get them to explain why

indicates that the starter motor is stuck in the ring gear
possibly faulty starter relay or starter solenoid
pull the starter as the pinion over ride clutch is probably damaged ( noise ) and have the start circuit and relay checked
have the starter bench tested for serviceability by an accredited auto electrician

In KD-DV5300, replace capacitor C751. Capacitors dry with age.

No Power? Do you mean that the engine would not crank when you turned the key? Did you have dash lights? Dome lights? If you had some kind of lighting, you had electrical power. and if that is so then it sounds like your transmission range selector switch may be faulty or needs adjustment. Or VERY POSSIBLY, you might not have put it ALL the way in Park when you turned the car off the last time. If you try it now does it start? If not try putting it into park a little harder. Does that work? If not put in Neutral and try it. If that works the Gear selector switch Is bad or in need of adjustment.

That is a false statement

You can put an Ampere Meter on it, when it is running

You can look at the motor running with an Oscilloscope

You can check Fuel Pressure & Fuel Volume

Your problem has nothing to do with cranking
Cranking is the starter turning over the motor

You have a no start condition, due to lack of fuel pressure

Now you see why I never recommend a dealer
Why would a professional tell you that stuff?

You have a complete fuel pump circuit with other items
that can be checked

You could install a new pump & hope that is the problem,
but if it took 6 months to come back I doubt it

Check for fuel pressure and check for spark during the no start condition. Also check the crank sensor, it is located behind the crank pulley mounted in the engine block on the 4.3 L engine
I hope this helps.