That is a ground controlled circuit. You would need to tap the ground wire of your cap lights into connector C1 terminal A2 of the I/P relay block located under the I/P to the left of the steering column, by your left foot. It is a gray connector.
<TABLE width="100%" border=1><A name=tab30-1243954><A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=1243954&pubid=437&cellId=62019#tab30-1243954" target="_blank">
<H4>Relay Block - I/P C1</H4></A>
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<TR>
<TH vAlign=center align=middle colSpan=4>

</TH></TR>
<TR>
<TH vAlign=center align=middle colSpan=2>
Connector Part Information</TH>
<TH vAlign=center align=left colSpan=2>
<UL =BULLET>
<LI>12193905
<LI>68-Way F Metri-Pack 280 Series Flexlock (GRY) </LI>[/list]</TH></TR>
<TR>
<TH vAlign=bottom align=middle>
Pin</TH>
<TH vAlign=bottom align=middle>
Wire Color</TH>
<TH vAlign=bottom align=middle>
Circuit No.</TH>
<TH vAlign=bottom align=middle>
Function</TH></TR>
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<TD vAlign=center align=middle rowSpan=2>
A1</TD>
<TD vAlign=center align=middle>
LT GRN </TD>
<TD vAlign=center align=middle>
24</TD>
<TD vAlign=center align=left>
Backup Lamp Supply Voltage - BCM</TD></TR>
<TR>
<TD vAlign=center align=middle>
LT GRN </TD>
<TD vAlign=center align=middle>
24</TD>
<TD vAlign=center>
Backup Lamp Supply Voltage - Junction Block - Rear</TD></TR>
<TR>
<TD vAlign=center align=middle rowSpan=2>
A2</TD>
<TD vAlign=center align=middle>
DK BLU/WHT</TD>
<TD vAlign=center align=middle>
149</TD>
<TD vAlign=center>
Cargo Lamp Low Control - Cargo Switch</TD></TR>
<TR>
<TD vAlign=center align=middle>
DK BLU/WHT</TD>
<TD vAlign=center align=middle>
149</TD>
<TD vAlign=center>
Cargo Lamp Low Control - BCM</TD></TR></T></TABLE>