This is actually a very simple job. However, there are some tricks to makes things easier and faster. The biggest thing to be mindful is that the sat antenna wire is VERY fragile. It's not a single copper wire. It's actually a miniaturized coaxial cable (think cable tv wire), so any kinks in the lines or excessive strain will damage the cable.
You know when your antenna is going bad when you start getting either connection problems, or it has trouble acquiring signal, or fades in and out of getting signal. One most common cause for antennas going bad is antenna wire damage from an improper install. The other is moisture or rain getting inside the antenna and eventually corroding the electronics on the inside of the antenna. This is more common than you think. We would replace a couple a week.
I worked as an Geek Squad Installer at Best Buy for about a year and a half. I'll pass along some tips so you can get this done fast and the correct way, even if it's not a Subaru you're installing this on. Running a replacement antenna should only take 10 to 15 minutes, but probably more realistically an hour for someone that's never done this kind of work.
First thing you need to look at is where you want to install it. The roof is the best place. It will get the best, unobstructed signal with less risk from damage from outside sources. However, some vehicles don't have the rubber "tract" where you can hide the wire. In cases like this, or for people that want to avoid messing with the vehicle's gaskets, the trunk is the next best place. Avoid installing this anywhere in the vehicle. It's okay if it's a GPS antenna and there's still a clear vertical view of the sky, but sat radio signals are just too week and the glass interfere too much.
An antenna at Best Buy retails for about $30. So if you can find it cheaper elsewhere, go for it. I'd advise against any used antennas. Again, the wire is very easy to damage. Try to find one with the little rubber feet on the bottom. This will help keep the antenna from scratching the crap out of your paint. Newer antennas have this. Older ones do not. They come in all shapes and sizes depending on when it was made. Electronically, they're all the same. With that being said, an XM antenna will work in place of a Sirius antenna and vice versa.
Here is a photo of where my old antenna was installed. I will be installing the replacement in the same place. Be sure to clean the area thoroughly with alcohol. If you bought one new, it should come with an alcohol pad. Otherwise, denatured alcohol and a clean cloth will work just as well.
Here's where I removed the old antenna. You can see the damage from the older antenna scratched up the paint. The dirt and residue removed easily with alcohol
Here's what comes with the antenna kit - antenna, alcohol wipe, and a stick-able rubber conduit.
Easy way to untangle your antenna wire. Remove the twist tie. Stick it to the metal of your garage door. Then start untangling.
Rough fitting on the car to make sure it's centered. Don't stick the rubber piece on yet! Be sure to leave a couple millimeters of extra wire so it doesn't tug on the wire slightly to one side when you start running it. It's ugly and not professional. Small detail, but small details make the difference. What you end up doing is if you run the wire to the passenger side, that extra few millimeters will make a loop to the driver side as basically a small strain relief (all this is under the rubber rain molding).
The reason for the rubber conduit is to extend the antenna further onto the vehicle. This is because the antennas don't work very well at an angle. If you're installing this on the trunk, you can chuck it. If this is being installed on a car with a low angle, you can chuck it too. Most trucks need it and sometimes we'd put two in series since the slope would be so aggressive.
This is a Bojo. They come in a variety of shapes. Most of mine was lost or broken during my time of working there, so this is the only one I have left. It's a plastic tool car electronic/audio installers use to run wire, pry, etc. It won't scratch your paint or interior trim. If you do not have one, you can use any hard piece of plastic that serves the same purpose. A tooth pick could even work in a pinch. Just DON'T use a metal flat head!
Use your tool to life up on the rubber trim and stuff your wire down in.
Rather than running the tool against the wire all the way down, the wire is stiff enough to just push and roll it's way to the edge of the car.
This part can get a little tricky. Doesn't have to be perfect as long as it's all underneath. I made another loop so there would be less strain on the wire and avoid weird tugs on the wire.
Run it along the next channel to the bottom.
The alternative is to place it on the trunk here. Be sure to center it. In a lot of cases, it's easier to run it along the trunk because you can hide it under the trunk trim (Subaru's didn't have any on my year). Then you follow the trunk wiring along the hinge.
If you're installing this on an SUV, be sure to make sure it won't interfere with your roof rack! In cases like these, we'll install it on the front of the roof. On trucks, top of the cap and have it enter from the 3rd brake light.