no power to head unit

I have a 1988 Jetta Coupe and I'm trying to put an IDA-X200 head unit from alpine. The stock stero had already been removed and an aftermarket put in when i bought the car that worked fine but couldnt connect to ipod (which is why i bought the new head unit). This is my first head unit install so if i made an incredibly basic mistake bear with me. When i pulled the head unit that came with the car, there was no wiring harness and it was a disaster of wires, i initially just connected each wire from the new head unit to where it was on the old one but when it came time to power up i got nothing. I then found a 12v constant in the ignition, tried that still no power, next i just tried to power the head unit straight from the battery and still nothing. i even took the old head unit (using the wire harness that came with the new one) and couldnt get power to THAT one. I'm at a loss so maybe someone can give me some insight on this. Thanks in advance for ANY comments

You are powering both the red and yellow wires, aren't you?

yellow needs to go to a constant 12v source and the red needs to go to a wire that reads 12v only when the car is on. black needs to go to ground. you might also want to check your fuses to make sure nothing is blown since you said the old wires were a birds nest

its the strangest thing in the world, i think it had to be the wire harness on the new head unit because after hooking up the old one with all the original hack job wiring the OLD head unit works fine but wouldnt get any power with the other wire harness. its an IDA-X200 and the only difference between the old wire harness and the new one is the yellow has a 10 amp fuse in the wire and the old one doesnt, i dont see why that would be an issue since every battery in the world is 12v but maybe i need a bigger fuse?

fuse size doesn't have anything to do with voltage. was that fuse blown? if so, thats your problem. or maybe there is a bad connection inside the fuse holder. you need to check these things. power problems are very simple to solve because there is only 2 things that could be the problem. either you have no power at all, this has been ruled out because the old radio powers up fine. or, there is a bad connection. and since the only difference between the new radio and the old is the wire harness. and since the wire harness is just wires except for the fuse holder then it is the obvious next step to check the fuse holder or make a new one. you can use 2 normal size speaker connectors and a regular buss fuse and some electrical tape it will look like this:

^^^^^after you make that make sure you tape it up and if that doesn't work, then the last thing you can do is run a new ground to bare metal. if you have a good ground and power on both red and yellow wires and it still wont turn on i would suspect you have a bad unit

thanks so much for all the replys turned out to be the fuse holder all is good now!