Do the wire colors on the harness that you snipped out of the 93 WRX match any of the colors in my pin-out table?
http://ae64.com/WRX-pinout.htm
Which pins are different and what are their colors?
Next, do the factory wire colors in your Legacy match Vikash's information here?
http://www.surrealmirage.com/vrg3/bcstereo/
How many wires are on the two harnesses in your Legacy? Did it total 15? (Legacies apparently have a +12V Accessory wire on both harnesses, and you only need to use one).
You'll just need to make a table. Make a column with the 14 functions (Ground, +12V Battery, Power Antenna, etc.).
In the next column, list the factory wire colors in your Legacy. Use Vikash's table to look up (either by color, if your car matches his table, or else by pin number/position).
In the third column, list the wire colours on the WRX harness. Use my table and go by pin number/position.
Then connect the wires from column 2 to column 3.
(this is a good reason to just buy the installation harnesses -- you'll have matching aftermarket colors, and you can do the work inside your house instead of reaching into the dash)
David C.
1996 SVX L AWD Laguna Blue Pearl