You might want to look into a "Y" harness like this:
http://www.ezonpro.com/products/familyVehicle/86yHarness.shtml

http://www.ezonpro.com/instructions/EZONinstructions_familyVehicle_86y.pdf

  I installed 3 in the back bench of my mulitivan. About $55 each plus shipping. They mount with one bolt in back on the deck (removable teather clip) and utilize the existing lap belt. I also use a cheap booster.
  I never had a problem with the lap belts until I read the thread about the kid that appeared to have slipped out of the lap belt in a violent collision on an icy road.  My kids had just grown out of car seats and they seemed a little too free and loose with just the lap belts.
  Good Luck,
  Chris C 90Multivan

  Mike Collum <collum@VERIZON.NET> wrote:
  This is the 3 point belt that I installed in my '85 Westy so I can
transport my grandson. It was quick and easy to install and it works great.

http://www.gowesty.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=KT-122&Category_Code=

Mike

Neal Barrentine wrote:
> Greetings fellow list members:
> The State of Indiana passed a law requiring children under the
> age of
> 8 and under a certain height and weight to use child booster seats
> while traveling in a motor vehicle. Aside from this legislation
> assuming that I don't know how to care for my children, it also doesn't
> take into account that my '86 GL has no shoulder belts in the back
> seats with which to secure a child in a booster seat. Every child
> booster seat I have seen requires a shoulder belt. The front seats
> (with shoulder belts) are already spoken for. I imagine that newer
> vehicles have shoulder belts for the back seat guests, and even
> convertibles have shoulder belts, so there has got to be a way I can
> put them in at least one position in the back of my Vanagon, preferably
> the seat behind the drivers seat. Any experience or suggestions? Thanks
> for reading this. Have fun, Neal
>
> Neal Barrentine
> Bloomington, Indiana
> 1986 GL
> 1960 Baja Bug
>