95 SLS Power Point
 

The '95 SLS I just bought doesn't have a Power Point to plug in for Cell Phone recharging or other electronic devices. I use my laptop extensively for my job as a Real Estate Appraiser. Since I don't have enough battery life, I need a way to provide an electrical source to power my laptop. I've tried different mobile AC converters that plug into the cigar lighter. However, I've noticed that it shuts off in about 10 seconds. Is this an inherent design feature of the lighter socket? If so, is there some way to stop/disable that feature? Is there another way (inexpensive/easy way) to provide DC power to the interior of the car?

Any help anybody can provide would be greatly appreciated.

Thanks