my bro had some cheapo things. The connected down where your seatbelt is now, feed around the back, under headrest and split into 2 to make harnesses. He never actually got round to fitting/using them
 
iv seen some things like that before but im abit unsure about how good they would work wunded what people thought if i connected them to runners in someway
 
NO! never do them upto the runners, it is dangerous and illegal!

The main straps should be close to horizontal as you can get them

something like this
man_side.jpg
 
okay i wont thats that idea out of the widow then not sure if theers away of doing it then without removing the back seats
 
Harness' will probably annoy you more... Have you thought of a CG lock?

I had harness' in with back seats in my old car. Had them as clip in clip out. Then I had them onto my iso fixing point (for baby seats). Probably wasn't safe at all, but it passed the mot so... Then when anyone wanted in just unclipped them and used the standard seat belt that I kept in. You should probably look for 4 points though as the iso points are a bit off centre of the front seat.
 
Schroth do 'trackday' harnesses that can be fitted and removed by itilising the existing seat buckles. They are the only ones i would trust.