The way I started was I found someone that worked in the business center and spent more time than I needed doing a detail for them. It gave me exposure. After I did the detail, I would come and wash their car every week. That way I got recognition and people started giving me keys to wash their cars.
The biggest thing to do is to have a sign, come regular, and provide quick service. In business centers, a lot of times people are in & out. One place I do is a physical therpy joint and the customers are only there for 1-2 hours. I knock out a quick wash and mini interior.
To answer your question, I would start by just walking into the office building on Thursday and telling them your washing cars in the parking lot and that you'll be here ever Friday from 12-5. Leave a flyer that states this along with your card and ask them if they can post the flyer in the break room. That way it gives them time to let people know and get keys from everyone. Show up on Friday even if your not sure that you have anyone and just stop back in and check with everyone. You might not get anyone at first but the more you show, the more likely you'll finally get someone that bites. This worked really well for me at a local shipping/trucking company. I started by washing my friends car and hung up a flyer saying I'll be here ever other Friday and now I get a minimum of 7 washes per visit. On top of that I up sell monthly wax and mini details. I keep my time invested to a min and focus on quick and clean not thorough and perfect.
I would also leave a coupon with the front desk that offers 10% off if you get 3 or more cars to wash. Also offer a deal to the secrutary and tell them you'll give them a free wax if they post/get you some business.
The biggest thing with office parking lots is getting your foot in the door. Once people see that your their, and see that your coming regularly, they will usually hire you. Another thing is to go and hit up every office in the building. I can't tell you how many times after talking to people and me coming for 2-3 months 2 times a month they tell me they had no idea I was washing/detailing at their work. Don't assume people know that you are there to wash anyones car and your not there just to service that one car your washing. Signs and talking are your biggest friends.
Hope that made sense and kinda helps. Good luck!