Let me update this to say:
This is an economical ($25-30 part), easy & fast (1-2 hrs to stick-on & wire into tail brakelights/harness) for a quick 3rd Brake Light addition. This light unit is enclosed with wires & adhesive face to mount it to your rear window - no screws nor clips needed to mount the light & no reflecting glare off the glass.
You'll need to look at your 914 wiring diagram to see where best to wire it, but they can be dropped down behind the rear backpad & into the trunk &/or engine bay to tie into the brake lights & pull off any easy existing screw to body for ground.
This part is probably also available elsewhere - maybe cheaper - & is on sale in Jan 2015 at GoWesty (they put them on sale a couple of times a year as Monthly Specials).
There are also several other more expensive & complicated solutions discussed below & with links by other posters, if this doesn't suit your liking. .....
This should also work well for those who want to add a 3rd brake light to their 914 at reasonable cost, but you may need some extra wire to reach the taillight buckets/harness to tie into.
http://www.gowesty.com/ec_view_details.php...gory_parent_id=
Click to view attachment
I put it on our `88 Westy as per their instructions at the GoWesty website after my wife had gotten rear-ended in it a couple of years ago, & it works well & adds visibility to our "brick on wheels"!
Happy New Year!
Tom
///////