Vehicle Speed Signal (VSS) and reverse gear can both be found in the passenger footwell, in locations similar to those shown here:
Forester 2009 / Impreza 2008 wiring for VSS, Rear Gear and +12V Ignition On
Applies to 2008-2014 WRX and STI, 2008-2011 Impreza, and 2009-2013 Forester
In Impreza/WRX/STI, you can see the connector with the VSS wire without needing to remove any trim panels. Crawl in to the passenger footwell head first, put your head where a passenger's left ankle would be, and look straight up. You should see the multi-pin connector shown on the above Forester thread. Factory wire color is PINK/GREEN.
Reverse gear signal is behind the kick panel (right side, near the door hinge). You may be able to spot the harness connector without opening anything, but you'll have to pull the panel to actually reach the wire.
On 2012-2013 non-nav Impreza (not WRX or STI) and Crosstrek, VSS is pin 19 on the factory 20-pin radio harness. Reverse gear is pin 16 on the 16-pin aux harness. Parking brake is pin 15 on the 16-pin aux harness.
It seems that nearly all recent Subarus have the reverse gear wire in similar spots in both footwells (going to the respective rear light clusters), and the factory wire color is always BROWN/YELLOW.
On models with navigation, the wires for VSS, reverse gear, and parking brake are all on the head unit harnesses.
ADDED: When talking about wire colors, the first color is the main or dominant color of the wire. The second color is the stripe(s). So PINK/GREEN is mostly PINK with GREEN stripes. On factory wiring diagrams, that's shown as PG (no slash).
List of Subaru's abbreviations here
(first one I could find quickly. Note that it's not
safe to assume BLACK is ground, especially if there is also a stripe).
Factory wires frequently also have dots. Those are usually not mentioned or documented.
ADDED: There will often be factory wires with the same colors, used for different signals. It's important that you only look in locations where you know the desired signal is near, either from a post like this, or by checking the wiring diagrams, looking up a harness connector number, and seeing the location shown in the drawings towards the end of the wiring diagrams (e.g., "Bulkhead Wiring Harness (In Compartment)").