What is the ambient temperature used in RTCA/DO 160?
Ambient is the conditions in the test lab.  DO-160 defines it in sec 3.5:  


  3.5 Ambient Conditions
  
  Unless otherwise specified, all tests shall be made within the
  following ambient conditions:
  
  a. Temperature: +15 to +35 degrees Celsius.
  
  b. Relative Humidity: Not greater than 85 percent.
  
  c. Ambient Pressure: 84 to 107 kPa (equivalent to +5,000 to -1,500 fi)
  (+1525 to -460 m).
  
  When tests are conducted at ambient conditions that differ from the
  above values, the actual conditions shall be recorded.


So anything within this range is acceptable and if it isn't within the range, just document what it is.  

You'll also note that in each of the temperature profiles, the test starts at ambient at T0 and shifts to the first controlled test temp at T1. DO-160 includes the following Note as the time to transition from ambient to the test temp will vary:


  1) Temperature rate from T0 to T1 is not specified.


You have to remember that DO-160 is about testing extremes. If the equipment can't operate at any point in the ambient range, it's unlikely to operate at the extremes.  

BTW, the latest version of DO-160 is rev G.  You should use it for new products.  Using rev D would only be appropriate for re-testing of a product originally developed to DO-160D.