Patio doors are actually very large windows and even when the weather-stripping and caulking is in good shape, they can create cold drafts in a room simply because there is so much cold glass that the air at the top begins to cool off, slides down along the glass getting cooler and cooler as it goes. Sometimes you will swear that the wind is blowing in through the wall, but it is actually an indoor air current. This often leads to condensation at the bottom of the door because unlike most windows in the house which have heaters under the window, there is usually nothing to counteract this dropping cold air under most glass patio doors.
Yes heaters for patio doors do exist, but are often difficult to locate.
The simplest to install and use is actually a baseboard heater built sturdy enough to walk on. Of course this only works if your patio door is at least 4 inches off of the floor. They come in different lengths and powers, all 220 volts and should be installed by an electrician. www.Ouellet.com
My favorite is the Qmark fan powered "Drop-In" floor heater that you see in the second photo above. This is designed to drop right into the floor between the floor joists. The box measures 14-5/8" x 7-1/4" and is 6-5/8" deep. You can use the built in thermostat (adjusted by a screwdriver through the grill) if you just want to keep the chill off of the patio door or a wall thermostat for heating the whole room. It has a built in fan that produces a flow pattern of warm air five feet wide four feet above the floor, preventing the cold air from reaching the bottom of the door and a warm air flow rather than a cold air flow near a patio door. Actually it looks and feels like you have a forced air furnace. The heater and fan unit can be unplugged from the black box and removed from above for cleaning. These are 240 volt heaters and should be installed by an electrician.
Because it is on the floor there are some limitations to its use. Do Not install in bathroom or other areas where water may enter the heater unit. Do Not locate where air flow may be obstructed (under drapes, behind doors, etc.). Keep short side 36 inches minimum and long side 3 inches minimum from side walls. Do Not install in ceiling or wall.
The Qmark FDI drop-in heaters are actually difficult to locate. The manufacturer's web site is www.QMarkmeh.com: click on Heaters, then Specialty, then FDI-Floor Drop-In Heaters -- there is a dealer locator on their site. You can also buy them on the web: www.ShopFnc.com: search QMark then Specialty Heaters then Floor Drop In Heaters.
For an article on adding insulation to a drop in floor heater, click here.