Just go to any dodgy KTV.  :P

saddle cantina, 1st floor.

http://www.echinacities.com/beijing/city-life/five-fantastic-mexican-restaurants-in-beijing.html

I found this on echinacities: 

Five Fantastic Mexican Restaurants in Beijing

Mexican Wave                                                                                                                                                     “The wave” is one of Beijing’s oldest and best kept secrets. Established in 1988, it is one of the oldest and longest owned expat restaurants in the city. The luridly bright colours paired with various objects hanging from the walls and ceiling makes the place feel a little cluttered but nonetheless a warm and homely atmosphere prevails. As well as the usual tacos, nachos and fajitas they also offer a large selection of pizzas and a children’s menu which offers advertised dishes in smaller portions at half the price. But what this place is really famous for is its inexhaustible list of tequila (check their website for more information). So if you are in the mood for a few shots, some quality food and decent company then head to the wave.

Add: Dongdaqiao Lu, Chaoyang District (East Side of Road 200 meters north of Guiyou department store opposite Silk Market), Beijing
地址: 北京市朝阳区,东大桥路东边 (秀水市场对面贵友商场大厦往北200米 )
Tel: 010 6506 3961
Opening Hours: 11:00-00:00
Price: 250-300 RMB
Website:  www.mexiwave.com

The other four are listed on the link I provided for you at the top of my post!