I have two seperate external switches near the rear door, one for the loading lights on the roof above the rear doors and the other for the horse area. Can I rig these up so that I can fully operate them from both the existing external switches and a pair of added switches in the living area ? Ideally I would like to be able to turn them on and off from both locations.

If you ask for a Single Pole Double Throw (SPDT)Toggle switch, you'll get what you're looking for.

click here >>  http://www.grainger.com/Grainger/items/2VLT9

As wyndancer said, you'll have to replace the existing switches and pull three wires to the 2nd location.  The below is copied from http://www.1728.com/project2.htm

Diagram 4 (below) depicts what is probably the most common use for the SPDT switch - the 3 way light switching circuit. Electricians incorrectly call the SPDT switch a "3 way switch". The proper terminology should be "three terminal switch". However the term 3-way switch has stuck and it's a misnomer we'll just have to live with.

So, how does this work? Let's say that Switch 1 is at the bottom of a stairway and Switch 2 is at the top. Suppose Switch 1 is in a 'down' position (B & C connected) and Switch 2 is in an 'up' position (D & E connected). The light bulb is off. Now someone comes to the bottom of the stairs and flips Switch 1 'up'. If you follow the circuit you can see why the light bulb would now turn on because A & B and D & E are connected. When the person reaches the top of the stairs, Switch 2 is flipped 'down', E & F are now connected and so the light bulb goes off. Another person shows up at the bottom of the stairs and flips Switch 1 'down', connecting B & C thereby turning the light on again. The person reaches the top of the stairs, flips Switch 2 'up' connecting D & E and the light bulb goes off. Notice that in the case of the second person, a downstroke turns the bulb on and an upstroke turns the bulb off. If you have such switches in your house OR if you have purchased household wall switches for this circuit, you now see the reason why they do NOT have the words on and off printed on them.

Hosspuller,does that switch come in 12 volts ?

sidelock

I just thought I'd say that if you are going through the trouble of rewiring everything, why not rewire it with the 3 way switching?  I think I could give you a little help. 

That diagram and explanation are from my website. 

Sidelock.  It is not complicated at all.  The hardest is figuring out which of the existing switch wires is the supply or hot lead.  Using the above diagram, label the supply wire "B" and the wire to the existing light as "E"

The toggle switch in the Grainger link is a three position switch.  The center position is "OFF"  This is useful for locking out the whole system.  If you leave the inside switch on center, nobody can turn the lights on from outside and run your battery dead.  Otherwise you'll just have to remember to move the toggle lever fully to one side or the other.

The switch is a good selection for a few lights.  Notice the rating is lower for higher voltages.  For 12 volts it'll be good for 20 amps.

Next, run three wires to the new location.  You'll want to use at least a heavy gauge wire as the existing wire.  Bigger is better since you're adding distance to the circuit.  But not so big as to be unmanageable.

Connect "B" & "E" to the center terminal of each switch respectively.  finish the other connections and...

Voila' you're done !

Note to Wolf1728 ... Thanks for the use of your diagram.  It is very clearly done.

Hosspuller, I'm not sure we are on the same page .I thought the switch in the diagram is a three wire , two position ,on/on switch and does not have a middle or third position.

Sidelock.  to reiterate...  The toggle switch in the Grainger link is a three position switch. 

It is electrically the same as the switch in the diagram.  You don't have to be concerned about polarity since you will be switching an existing circuit.  The lights are already negative grounded.