Why not read the 12-page pdf installation manual from Subaru at
http://techinfo.subaru.com/html/ixSearchFree.jsp?text_keyword=&keyword=&publication_type=12&free_pub=13
2008 Remote Engine Start System (Legacy 08MY+) (H001SAG100) (1441.90 kb)
Wish I had to install one myself.
The remote starter duplicates the function of the key. You will need to get under the dash and solder the wires from the remote starter to the proper wire harness wires.
In newer vehicles you will also need to properly bypass the car security features that are usually part of the key.
You will probably need to install a pin switch to prevent the remote start from starting the car with the hood open.
Lastly you will wire to power/ground and the brakes.
in the engine compartment
On the top at the rear of the engine
Has a starter relay in the engine bay fuse panel.
On a 1996 Subaru Legacy, the starter relay is located under the head in the relay box. The relay box is located on the driver side at the rear of the engine.
left side engine compartment . follow battery cables to the starter
The 2001 Subaru Legacy has a flat 4 engine.
The 2014 Subaru Legacy has a flat 4 engine.
The 2009 Subaru Legacy has a flat 4 engine.
The 2013 Subaru Legacy has a flat 4 engine.
The 2010 Subaru Legacy has a flat 4 engine.
The 2005 Subaru Legacy has a flat 4 engine.
The 2008 Subaru Legacy has a flat 4 engine.