connecting gps, radar audio to headphones

You are correct. But if you take a stereo's 2 wires and splice them together, you just combined those to a mono. if you listen closely to your tv, youll take notice the the right speaker and left speaker dont always have the same "sounds" all the time. When you listen to a music cd, some intro's to a song might only use the right speaker and then all of a sudden the left speaker joins in. If you were to disconnect 1 speaker, then youll lose what that speaker put out. In your case, if you combine the left and right, youll have both of those, but just in a mono state. You need to do this because you are going from 3 outputs (2 right/left from one and 1 right from another) but you are running those to 2 inputs (your left and right headphone speaker).So you will have 2 rights and 1 left. You need to "remove" the one right by simply joining it with a left to remove any chance of blending together. I will show you some diagrams.

You said your GPS has a stereo jack and the radar has a mono. That is what I drew above.

yes the gps has a stereo output but im plugging in a mono cable...did you see the cable with adapters that im using?  Im getting feedback through, I think the common at the stereo jack at the Y cable im using...so my original question is will diode isolating it solve the problem?

Well thats why you are getting feedback. You are taking a mono cable from each unit. All mono cables use either the right or left channel. So you are getting the same channel from both units. You need to seperate them. You need a mono for the radar and a stereo for the gps. You then splice the stereo together, wire that to a stereo wire that you have plugged to your headphones. Youll cut the jack opposite the one connected to your head phones. Youll then expose the 2 wires. The one wire will go to the gps  the other will go to the radar.